It was originally proofed as a 26 bore. ( .563).

I have not found any reference to 26 bore cartridges being made.

It might have been made as a open bored 28 bore, or a tight bored 24 bore in the way that new 12 bores were often stamped 13, or 13/1.

If it was chamber sleeved from 24 to 28 you would not see any external joins, only possibly a faint circle in or around the cartridge rim recess.

In any event it is now nitro proofed as a 28 , so I am sure you will continue to enjoy shooting it.