Any question answered...

OD of the seat tube is probably 31.8 (i.e. 1.25" nominal), wall thickness is likely to be 0.7 for single butted tube or 0.8 for double butted, so 30.4 or 30.2 seat post. I have a 30.4 you're welcome to try, just a basic Kalloy thing.

Thanks for the offer, let me see if I know anyone with a seatpost measuring tool first, when that fails I'll get back to you.