Dear Campero,

Are there any earlier Black Powder proof marks stamped on the barrel? Does that show a 24?

My guess is that it could have been made as a 24 bore, and when those cartridges became unobtainable then the chamber was sleeved to accept 28 bore cartridges.

I know that some gunsmiths chamber sleeved 16 bores to 20 bore to make them more saleable.