Oops! I'd forgotten about this auction on ebay which had ended yesterday. To be honest, it sold for more than I would have bid for a frameset which was in immediate need of considerable external rust removal, and perhaps even a full restoration re-enamel before I'd have felt comfortable exposing it further to the elements.
I'm a bit curious about the year - my guess would be around 1964-65. I have one which had similar blue paint and lug outlining which I seem to recall had been estimated as perhaps around a 1968 model?
If anyone is interested in this sort of thing, the frameset at auction has some interesting and rather uncommon graphics on the seat tube. Nicer (in my opinion) than the later 1970s models.
Here is the link to that auction page (item 250007230498):
You may note also that this frame still has a rivitted metal headbadge - which mine did not. I found it interesting that my frame, once stripped of paint, clearly revealed a set of previously drilled headbadge holes in the head tube which had been soldered over at the factory. So, it would seem that Peugeot had a fair stockpile of 531 head tubes or even complete framesets all ready to roll... and perhaps slower than hoped for sales during the mid-1960s.
Bob Hanson, Albuquerque, New Mexico, USA