Dwight,
Very interesting. This is the first example I've seen where the serial number and "Powells Patent" number are out of sync.

As I mentioned before, gun No.4488/Powells Patent 879 dates to 1870. Gun No.5015/Powells Patent 1345 was sold in 1873. Gun No.6098 has Powells Patent 1883 which I believe was sold in 1876.

The wedge fastener would seem to date it pre-1874.
Steve