Originally Posted by
Hondo6
Pretty sure 31.8mm is a standard seatpost size, just not terribly common.
A quick look on eBay shows a listing for a used Thompson 31.8mm alloy seatpost. So Thompson at one time appears to have made them.
There seem to be multiple listings 31.8mm seatposts from other manufacturers.
Since the frame is new, I'd try to get satisfaction from the vendor first. But if the insert is indeed reamed oversize and a 31.8mm will fit properly, that might also be an option.
The specs on my frame drawing say 31.6mm. I don't think that there is a standard 31.8 seat post. I think a lot of folks get confused as 31.8mm is standard clamp size for handlebars.