Originally Posted by Fidler

I have measured the post diameter with digital calipers just above the seat clamp and it read 27.2mm. Im not a DIY bike savvy
person so not qualified to remove the post and put it back again.

I find it interesting when you say that a 31.6mm post would never fit inside a 31.8mm seat tube.
Im pretty sure the tube is 31.8 as I just bought a front derailleur for those specs and it fit perfectly.

So a 27.2 mm post would fit in a 31.8mm tube?

The only mystery now is why my invoice for the existing oval seat post is 31.6mm.