I typically don’t use an apostrophe with plurals in any situation, but I always assumed that the use of an apostrophe in constructions like acronyms:
Forty BA’s were given out to students this year.
Though the greatest period instability occurred during the 1950’s when. . . .
was more or less ok. I would personally use BAs and 1950s here, but I've never thought the apostrophe in this case was necessarily “incorrect”. What say ye?