With dates like this, you don't need the apostrophe for "of". 1920s fashion = "fashion of the 1920s" You could also use '50s where the apostrophe takes place of "19" but I think writing out the year is clearer especially if you aren't addressing the very old. Neither of your alternatives are precise as written, though they might be understood with context. "a guitarist who was popular in the 1950s" "a guitarist who performed in the 1950s" "a guitarist who plays/played music from the 1950s"

I speak US English. If you mean "a guitarist who was popular during the 50s", then I prefer "a guitarist from the 50s" instead of "a guitarist of the 50s". "A guitarist of the 50s" could mean either a guitarist of the present day who plays music that was popular during the 50s or a guitarist who was popular during the 50s. You can use this with the name or description of any era or specific time span: "the 50s and 60s", "the Swing era", "the 19th century", and so on.

Hi I think your question is about the apostrophe. Yes you can use and apostrophe to replace 'of' - that's what these apostrophes do. "The house of Mike." "Mike's house." , I guess some people might argue that you would be better off writing 'fifties' and adding an apostrophe. And other people might argue that because you are actually writing 1950s and omitting the '19' you need an apostrophe before the 50 as well - so '50s' guitarist (personally I don't agree with this because 'fifties' appears in many dictionaries as a word meaning of the period from 1950 to 1959)... Exceptions? Not sure. I can't think of any situations where we might use this except for years.