Sticker says Reynolds 531? If so it should be 27.2. Maybe you should ask a qualified lbs or framebuilder to ream the seat tube. If you end up using too small diameter seat post it will move inside the tube and potentially cause cracks...
Not sure about reaming. I've been finding the old HQ frames have a mix 27.0 and 27.2 seatpost. I've 2 27.2 and neither are fitting so want to see what it looks like with a size down.
The old one that came out was 27.2 but it was a bitch to get out and stuck in
Sticker says Reynolds 531? If so it should be 27.2. Maybe you should ask a qualified lbs or framebuilder to ream the seat tube. If you end up using too small diameter seat post it will move inside the tube and potentially cause cracks...