The 26 stamp was applied about 75 years before the 20/oops!/28 mark.

The 26 in the Victorian marks refers to the bore size rather than a cartridge in the way that a .577/.450 Martini Henry would be 52.

The bore now is larger than 26 and is in the 24 range so it was clearly skimmed out to clean it up prior to being nitro proofed.