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@Rik_Van_Looy
^ 26.2 on the seatpost is an initial clue....
I think a hefty plain gauge 531 frame might possibly come out at 26.2, but 27.0/27.2 are much more common, is it a particularly heavy frame?
Whatabout the outer diameter of the seattube? Measure it, the standard is 1.125" (28.6mm) which would mean pretty thick walls with a 26.2 seat post
Unless it's French, a bike of that era would most likely be made from Reynolds 531 or Columbus SL or SLX.... the single-butted seat tube was 0.9mm thick at the bottom and 0.6mm thick at the top giving a 27.4ish opening into which a 27.2mm seatpost would usually fit.
For bigger frames, heftier tubes were used, like Columbus SP with 1mm wall thickness at plain gauge. This would mean 26.5 - 26.6 ish....
Curious.
It looks like an excellent frame, very elegant. Are the track ends marked Shimano?
That'd be another clue, I'm beginning to wonder if it might be japanese perhaps.....
Do you have a serial number on the frame? if so, where and what is it?