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@Dutch_Cheese
Usualy it's 27.2mm.
Maybe very old ones take a smaller post, but I'm not certain.
For sure since 1984 it was 27.2mm.
I think 753 has had different diameters.
Some exceptions: mixte frame, 70cm A frame, tandem, maybe some TT frames could be made for 26.xxx posts, because they've used plain gauge seat tubes for strength reasons.
Just have the seat tube reamed out at 27.2mm.
It will save you from a lot of trouble now and in future and it will not weaken the frame.