As was noted, the full 531 Peugeot with stock 27-in clinchers was the UX-10. I no longer have the catalog collection I once had, but my memory is that IF there ever was a catalog mention, it is VERY brief and at the bottom of an early 70s Peugeot pamphlet. I would suspect that it was a variant produced only for the U.S. market during the great bike boom - at least, that's my memory of the discussion several years ago, from before the bikelist.org days.
I'm always astounded by how many details and how much obscurity there is surrounding these bikes. I think it's been pretty well proven that they were spec'ed differently for different markets. Charlie Young's presentation of a PX-10E frameset with an odd mix of features has re-awakened a sleeping monster in my soul. Sometime in the next couple of months, I'll probably revive the PX-10 database gathering again. There are a lot more folks on the CR list now and with them comes a larger source of information.
Russ Fitzgerald
Greenwood, SC
http://internaldetours.blogspot.com