There’s no requirement for the apostrophe before the “S” in decade names like 50s and 60s, since there are no omitted letters, though it’s also acceptable to include one. The term may be written “’50s” since “19” is being omitted, but “50s” is fine too. Logically one should be able to use both apostrophes, writing “’50’s,” but this looks awkward and is seldom done. Personally I prefer to omit both apostrophes.
Writers who wish to have their references to decades clearly understood in the twenty-first century would be well advised not to omit the first two digits.
Note that you may have to turn off “smart quotes” in your word processor to get a leading apostrophe like the one in “’50s” to curl correctly unless you know how to type the character directly. Or you can just type two and delete the first one.
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