I am late to this thread, so don't know even if anybody will read this, but.....
From an English grammar standpoint, when 50s is used to describe an era, it is used without the apostrophe,as Ellen points out above, i.e., "This dress from the 1950s...." or "I put this as dating from the 50s."
But when using it as a possessive, i.e., the correct usage is, "1950's dress." (dress of the 50s).
So, depending on usage, either designation is correct. But I've found on eBay that if I don't use the apostrophe in titles, even if I should to be grammatically correct, I am in the "running" with far more items, and I think far more searches. Alas, people in general today ignore punctuation, and in eBay titles, I think try to save space by leaving them out. So, my feeling, correct or not, is that one will get more hits without the apostrophe....
(from an old-fashioned, , serial-comma-using, anal communications major!)