I've been poking around and what I think you have there is a 1977-79 PY-10/CP. In other words, the best version of their highest-end bike. Wow again.

531SL was supposed to have been introduced in '77 and the decal scheme changed in '80 so that gives you a decent year range. The PY-10 is the only thing which shows that tubing in my catalogs, though T-Mar has mentioned that the PZ10 and Pro10 used that tubing as well. I didn't try to figure out when those models were made. The crank and front derailleur have obviously been changed to something much newer but the rest of it seems to fit. You may even have better luck finding replacement original parts than we do over here.