Oh, man!!!
What is the size of the frame? Measure it from the center of the BB to the center of the Top Tube, along the seat tube. If it's a big frame, i might be interested. Post a pic if you can! you have a full 531, and a good one, at least when it was new. The Mafac Racers may have been installed later.
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Je vais à vélo, donc je suis!