The discussion of CPUSA membership after the Pact reminds me that it would be a nice project for someone to bring together documents or citations to documents that provide reliable numbers of CPUSA membership by year.
We have all sorts of estimates from the origins of the party to the 60s and 70s, but my impression is that for many years these are just estimates, educated and sometimes no-so-educated guesses based on limited or no ducumentation.
For some years, certainly we have reliable internal party records from fond 515 or party reports to the Comintern. The FBI also generated membership estimates, many of which are now public in FOIA files. Presumably the FBI estimates were based on information from its informants in the party in the post-WWII years or its surreptitious copying of CPUSA office records. But I don't think anyone has attempted to bring those all together and produce a documented membership record over a long span of years.
It would be a useful project for someone with the time and energy to do it.