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@Mr_Bump
you've cleaned the inside of the seat tube I presume, if you are not exactly straight with the calipers it can be so easy to get differing readings.
I've used a 50p plastic caliper from tesco, actually takes quite accurate measurements compared to my digi ones..lol
Okay, so what is the manufacturer/tubing (if known)
It could be anything from 26.0 to 26.8 though 26.6 is far less common
26.2 is fairly common for old single butted 531, 26.8 for some older double butted 531 (I had a '71 Claud Butler that size) 26.4 is relatively common for continental frames.
Do you have any old posts you can try or would a LBS measure it for you for nowt if you smile nicely :o)