Take a look at Sheldon Brown's French bikes page. Maybe this is a UE-18. I sold the UO-8 and PX-10 back in the 70s. The PX-10 didn't come in a mixte, and the UO-18 wasn't what I'd call lightweight, but if you substituted alloy wheels and swapped other parts, the weight would be more acceptable.

This page shows some randonneuring mixtes with fenders and uprights bars from the 50s.

What does the bike you found have for a crankset? Cottered or cotterless? Steel or alloy? Are there any stickers on the frame? You might expect a sticker with the tubing listed, such as "Reynolds 531" if it's very high-end, or something like "2030" if it's low end.