Possibly started life as a 28 bore made for thin brass cases, in which case the barrel could have been originally bored at 26 - based on the original proof marks.
When later submitted for re-proof (nitro) - the man with the hammer and stamps was obviously confused /hungover/or just having a bad day. Unless the nitro proof marks are spurious- which is not unheard of.
Presumably the barrel was built as a cylinder but does it show any alteration now?