Most French bikes used metric diameter tubing... 28mm seat and down tubes vs. 28.6mm Imperial or Inch size tubes.

The standard size seatpost for most metric seat tubes was 26.4mm. As rootboy suggested, you need to carefully clean the old grease and crud out of the seat tube. Also gently spread the ears of the seat lug a little with a large screwdriver or some other kind of wedge. After sanding the inside take a small round or 1/2 round file and make sure that there are no burrs inside the lug.

Before greasing the seatpost check to see if it fits first. I grease both inside the seat tube and the seatpost.

Chas. verktyg