Jim, I don't have a Chicago Manual of Style, but my copy of Turabian says that alphabetic plurals with apostrophes are an allowable form, as are alphabetic plurals without apostrophes, as long as one is consistent. Plurals of numbers, including dates (eg, 45s, the 1960s) are formed without one.

A good (and funny) book which examines the differences in transoceanic punctuation style is Eats, Shoots & Leaves by Lynne Truss. She says that "b's" is fine for more than one "b", and that "1900s" is normal everywhere but the US, where "1900's" is the accepted plural. The jury is obviously out on the latter.