In “The British Astronomical Association: The Second Fifty Years” publication (BAA Memoirs Volume 42 Part 2, 1990 December) on page 24, it says:
“… The Index to volumes 1-50 of the Journal was published in June . It had been compiled in 1940 by Mrs Reade who was then Librarian. Part of the Merlin bequest was used to finance its publication…”
Looking at the SHAs catalogue it looks like it is a separate publication, and not a memoir. The catalogue suggests there is a copy in the SHA library. I am going on Tuesday if there is something specific you want me to look up.
There is a printed copy of the index for the first 50 years of the JBAA. I had a copy which I sent (along with a large set of old JBAA’s) for the scanning project for the DVD’s. My old JBAA’s and the index were part of the library which Reggie Waterfield left me in his will. I got the JBAA’s back (these have now been donated to the BAA Archive with Richard McKim) but my copy of the Index was never returned. Sheridan Willams (who did the scanning) said that ther were only two copies in the BAA Library. It was pity that my copy never made it back to me. If it does turn up let me know.