Solunars by Aldebaran Sidereal Academy

Is there a written record, or otherwise, reliable historical information, about when exactly did Fagan and Bradley start communicating/corresponding?

Not to my knowledge. From what Gary told me, it wasn't too long before their triumvirate of books were taken up by the Llewellyn Foundation (all done by 1950).

https://solunars.com/viewtopic.php?f=19&t=178
This link is to an article Fagan wrote for The Astrological Bulletina for 1949. Bradley edited the Bulletina, so this shows correspondence interaction between them in 1948, which is the approximate time all the other facts point to (I would have said '47 to '48 based on the publication schedule). In this article, Fagan takes Bradley's earlier (standard Tropical) analysis of Griffith Abrams death and looks at it through the lens of solunars. By the time of this article, Fagan was already putting out his Sidereal ephemeris through AFA, and Llewellyn was distributing it. I have the '49 and '50 ephemerides, and the former alludes to the 1948 ephemeris, so Fagan was at least calculating some version of these ephemerides from 1947 (for the '48 issue).

The copyright on Solar and Lunar Returns was 1948. The Fagan-Bradley correspondence preceded that writing, which again makes me think 1947.

I should mention, though, an important part of the time line usually missed until you lay all this out: Even though Profession & Birth Date and Zodiacs Old and New came later (both with 1950 copyrights), the core work of both of those works had been done by the time Solar and Lunar Returns was published in 1948. How do I know? Because the VP tables in the back use the Spica = 29° Virgo standard. (VP 1/1/1948 5°53'21" Pisces, a reference to the VP crossing 0° Aries in 213 AD credited to Fagan). Fagan originally used Spica = 0° Libra (Lahiri standard) as a starting point, but moved this back to 29° Virgo (the value on which the Solar and Lunar Returns tables were based). Fagan's 1949 ephemeris still used the Lahiri standard (he gave Neptune as 21°59' Virgo on 1/1/1949) so, when he was calculating this in 1948, the 1° backup hadn't occurred yet; but it was already there in the 1948-published Bradley book (not sure what month it came out).

The history of that 1° switch isn't generally known. Bradley credited Fagan with it. I know through Gary (who was there at the time) that Bradley had already completed the Profession and Birth Date study (not published for another two years) which showed Fagan's first estimate was 1° off on the boundaries of the zodiac. Either this was the last clue Fagan needed to solve the Hypsomata problem or he had concurrently come to the same conclusion and not announced it yet. He replied to Bradley confirming the 1° shift (not sure how he wrote that, just that it was a basic, "Yes! You're right! This is exciting!" sort of letter.

So the two came to the same conclusion about the same time and were marching forward together on it from them on. This had to be sometime in 1948 and probably not early in the year, because the '49 ephemeris still had the old standard but the '48-published Bradley book had the new standard. (The one-two punch of ZOAN for the archaeology and P&BD for the statistics was published in 1950, side by side, to complete making their case.)

Fagan's first publication on solunars in the AFA journal was 1944 when Bradley was 19. I'm pretty sure he wasn't working for Llewellyn yet and don't know how deep into astrology he was; but he was writing for and editing the Bulletina by 1948. (Fagan had an article in the '49 issue.) In any case, he was aware of Fagan's earlier work by then.

So they may have had connection and interaction by 1947 but at least had it by 1948 (early in the year, probably).

Also, assuming such meeting did happen, when and where did he meet Bradley for the first time in person? (From what's available online, I know that Fagan moved to Tucson, AZ in 1966; there's no more detailed info about this than the year itself, and info of there being a note written by him dated April 1st 1966 with place noted Tucson, which indicates his coming there was sometime between January and March of that year.)

You've found the right spot. I know specifically that the two never met in person until Fagan moved to Tucson, where Bradley was already living (and working at Clancy Publications).

Confirming some small points of the above speculations: I pulled out the Sidereal Fixed Zodiac Ephemeris for 1950 by Cyril Fagan. In a short Foreword, Ernest A. Grant confirmed that the 1948 was the first in the series. His Foreword is dated November 1, 1949, so the ephemeris was not yet in print but probably was about to be. The publication was by Gestetner mimeo, so production process would have been to have all the rest typed, proofed, and ready to go, then add the Foreword page more or less right before they ran the whole thing (or they actually could have created that page after the rest was completely printed and ready to go).

So early November 1949 is when the 1950 ephemeris was released, so early November 1948 is a good estimate for when the 1949 ephemeris was released.

In the 1950 ephemeris, Fagan begins with an Introduction called "The Hypsomata," which (probably for the first time) publishes his discovery of the exaltation degrees mystery and the corresponding archaeological evidence for the correct boundaries (within 1°) of the zodiac.

In this Introduction, Fagan foreshadowed:

Readers will find this story of my discovery set out in detail in THE BEGINNINGS OF THE ZODIAC, to be published in January 1950, by Llewellyn Publications...

This, of course, was retitled Zodiacs Old and New. In the ephemeris Introduction he gave a short version of the discovery and some general arguments about how archaeologists and astrologers would now have to revise their theories on quite a lot.

For (probably) the first time, he wrote:

Fagan wrote:Years ago when I realized that Spica was the marking star of the Egyptian and Babylonian heavens, I naturally assumed... that it was placed diametrically opposite to the starting point of the zodiac, Aries 00°00', namely in Libra 00°00' (180), especially as this longitude - albeit polar - is given for Chitra (Spica) in the [various] siddhantas (the ancient Sanskrit authorities on astronomy). [Reference to Lahiri and others.]

Imagine, therefore, my surprise when I discovered that the priests of Nabu read Spica as being in Virgo 29°00', and not in Libra 00°00'. Moreover, they appear to have more than one marking star, for example, Aldebaran ("The Prognosticator", i.e., "Astrologer") hailed as the star of Nabu in Taurus 15°, the Pleiades in Taurus 5°, Regulus in Leo 5° and Antares in Scorpio 15°, measurements being made from whichever star was the most convenient.

He then explained (with apologies) that it was necessary in the new 1950 ephemeris to use a zodiac 1° earlier than in his earlier ephemerides, and people would have to subtract 1° from all the positions in the 1948 and '49 volumes.

Fagan and Bradley were both Taurus Suns. (The humor has not been lost on me that two Tauruses spent much of their life showing that Taurus is the first sign of the zodiac.) They both had such traits as devotion, earnest pursuit of truth, and passion for their causes. After that, there are stark differences - polar pairings - between the charts. For example, Fagan had a closely foreground Sun, Bradley a moderately foreground Moon. Bradley's Mars was in Gemini, known to be "scientific" primarily, while Fagan's Mars was in Pisces, marked most clearly perhaps by having more of an artistic than scientific temperament.

But the most vivid distinctions, I think, were in their Moon signs, Fagan in Virgo and Bradley in Aquarius. They both were responsive to and could get excited by intellectual thrills. The approach was a bit different, though: Fagan's Virgo Moon took him deeper into pure scholarship, especially through history and archaeology, with his greatest passions seeming to have been uncovering historic evidence, his commitment to establishing a stream of consistent historic facts. Bradley's Aquarius Moon was more drawn to pure science, technical breakthroughs and objective scientific examination, with his greatest passions seeming to have been letting nature disclose its patterns in objective data with a commitment to leave something behind for humanity as a whole.\

Both loved astrology. Both had troubled, ostracizing or bullied youths. Fagan was a social figure who (hey, he was Irish!) like to spend his nights at the local bar, loud and engaging, dragging someone off to a booth to read their palm or sketch a horoscope. Bradley was private, withdrawn, so that Llewellyn George originally had to be very protective of who got near him (since the LA astrological community loved the attractive, brilliant young man, but the crowding got to him badly) and only made rare exceptions, like when he introduced young Gary Duncan to him (it was unusual but he wanted the two young men to meet and work together). In his later years (he was only in his 40s when he died) he was gracious, but not the kind of person you could just drop in on. In hindsight, I'm surprised he took my phone calls at home at night - in hindsight, I would have expected him to want more privacy, - but he had a gentle voice and was easy to engage in conversation. He was probably quite emotionally vulnerable all of his life - this lasted to the end, and perhaps got understandably worse as he was in his last illness.

From the solar-lunar difference between them, I've privately thought (and probably mentioned to Gary) that with Fagan rightly called the "Father of Sidereal Astrology," Don was its mother; but I know that if I say that too freely, some people will understandably think I'm making a homophobic crack (which I'm not). Nonetheless, I think it describes their relationship with regards astrology in general and Sidereal astrology in particular.