The Lexicon is very consistent in all of its pages, and says that Peter joined Voldemort's ranks in 1979. I definitely trust the Lexicon, but I can't figure out how they came up with that date, and why they're so sure it happened before the Prophecy. I had always assumed that Peter had joined shortly before betraying Lily and James, but I suppose he was spying for over a year when he did so.
Can anyone find anything in the books? I'm hoping to write a one-shot about this era, and I'm a tad confused as to the timeline.