What punctuation mark is used to indicate the unknown year of a decade?

Solution 1:

There are multiple ways to do this. TVTropes has plenty of examples.

Some works use an em dash:

On a fine May day in 197—, Fred Regent and Richard Jeperson stood in Old Compton Street, London N1.
_{Soho Golem by Kim Newman}

Another way (very popular in video games) is to use X:

The year is 199X — Earthbound

The Oxford English Dictionary uses periods (e.g. "14.." for the 1400s or "187." for the 1870s), which is something I haven't seen used anywhere else, especially since it would be confusing when written in a sentence. Still, it's worth mentioning. Here are some screenshots from their page for up (adverb):