this is really difficult to reply to.

You see when we supplied Peugeot we did supply tubes that were metric size
(28.0mm outside) and imperial (28.6mm outside).
Then in each diameter the thickness of the tube was also supplied, 0.7mm or
0.55mm.
The frame builder would also ream the inside of the tube, so really it could
be a 26.6, 27.0 or 27.2 depending on the seat tube and reaming done.