It refers to a particularly popular Potterverse fanfic. (See wikipedia on Harry Potter Fandom). Set in the marauder years which itself refers to about 1970 when Harry's parents would have been in school.
It's rather hard to track down online but here is a PDF adaptation.
ETA: There is now a third thread, so send new comments there.
Since the first thread has exceeded 500 comments, it seems time for a new one, with Eliezer's just-posted Chapter 33 & 34 to kick things off.
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