The Reynolds 531 70s Jeunet that I have (back-burner project) does not take a 26.6 mm seatpost, that's for sure. Right now, I don't have any smaller ones to try. Post opening measures 25.8 or 26.0 with a dial caliper; but it's also a tall frame -- 63 cm C/T, so perhaps it has an especially robust seat tube in terms of wall thickness. It's not a particularly heavy frame, so I do believe that it must be constructed of double-butted tubing, and it had Reynolds 531 fork decals that are "original," French, and worn nearly to nothing now; and a shadow where the R 531 decal would have been on the seat tube, in the right size and shape. I'm 5'-10, buy trousers with 32" inseam, and prefer bikes in the 60-61 cm size.