The tontonvelo and the Go-Go guys, for sure, will know. Reynolds 531 hit the market in '35, but I don't know when Peugeot began using it. I would love to see nice detailed pix of the head lugs and dropouts and any other interesting details. It used to be a common thing for people to strip down bikes to fixed-gear setups for training purposes, and this might well have been someone's older, replaced road bike repurposed this way.