Old-time percussion caps were very corrosive, and percussion rifles that were heavily used sometimes show erosion or burning of the breech around the nipple. this is not to mention any corrosion that might occur in the bore at the breech. It was not unusual for rifles to be re-breeched. James Josiah Webb described having his rifle, "Old Blackfoot," fitted with a new "breech pin." It is my understanding that this was typically done by cutting off a couple of inches at the breech and moving the barrel back in the stock.
I was thinking this rifle might have been originally fitted with some sort of patent breech. If it was cut at the breech and moved back, the smith who did it might not have had the wherewithal to fit it up with a new patent breech, hence the drum and nipple. He might have thought the "fence" that was originally present was a nice feature, so he left it. I can only speculate.
It would be hard to say, until an expert examines the rifle "in person."
Best regards,
Notchy Bob