That Reynolds sticker does not say "531R" nor does it say "double butted tubes, forks, and stays." So if the sticker is original, I can't imagine the frame actually being 531R, which was nearly the top tube set. As it is, it states that the main tubes are 531 and that they are butted. Most commonly the TT and DT are double butted, and the ST is single butted. The forks and stays might be any kind of steel, and might or might not be butted. They could be CrMo and butted, or they could be lowly 1020 and not butted. We won't know.

With those dropouts, I think it's '90s.

I think it's a decent steel frame, midrange overall. It could ride extremely well or very poorly. If you don't have an everyday bike, this is worth building up.