It really is a very attractive bike. Nice window work on the lugs, and the lines are pretty. I'd be tempted to keep it if it was the next size smaller.
With regard to the 531 sticker, I did a bunch of research on Frejus frames of this era and that's where the factory apparently put them. It takes a 27.2 seatpost, and there was a square area on the top tube where there had obviously been a sticker before, so I bought one that looked like the closest to original.
__________________
Hard at work in the Secret Underground Laboratory...