It's probably a cheap steel open pipe post that fits in the insert you can see in the pic. Remove the insert and measure the inner diameter of the seat tube. You might get better info on this in the C&V forum.
FWIW, I have two 1970s French bikes, and they variously take a 25.0mm post (Gitane Interclub) and a 25.4mm post (Mercier 100), but the Peugeot could be different.
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Last edited by TejanoTrackie; 09-20-10 at 09:11 PM.