This is what I know so far.
Bores are 16.1mm which is .634"
They measure .645"
20 gauge is .615" and 16 gauge is .662"

20 gauge chambers are .698" at the entrance .685" at the cone.
These are .705" entrance, .695" at cone.

So the bores have been honed about .011' from the original proof marks. of 16.1mm
The chambers have been honed about .010" from nominal 20 gauge chambers.
So, I think it's a 20 gauge, that has had the chambers and bores honed.