I suspect a difference between American and British conventions here. I've noticed, especially when discussing recordings, Brits tend to write 45s, 78s, LPs, CDs, the 1950s, and so on, while Americans are likely to use an apostrophe: 45's, 78's, LP's, CD's, the 1950's.

Of course, we Americans don't generally use an apostrophe with plurals; this convention applies only when the thing being pluralized isn't an ordinary word, but rather a letter, a number, or an abbreviation.

Although I'm American, I have recently adopted the British style of writing CDs, 1950s, etc., because it just makes more sense.

Dang it, I've got a Chicago Manual of Style around here somewhere; if only I knew where to lay my hands on it!