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.