Did some googling last night, seems like the guy was known for making light guns...
- The address of "172 New Bond Street " was used between 1892-1893
The proof mark crown over BNP, was used the Birmingham proof-house starting in 1954
The diamond with 12 over C was used for the chamber size, from 1887 to 1954
The diamond with only 12 inside was introduced in 1954
The 2 1/2" was introduced in 1925
Just thinking out loud, but in the 1890s, wouldn't the barrel had been some sort of Damascus?
The "L" to the left of the crossed swords, (bottom left of the barrel flats) would indicate a 1960 reproof. Having the gun proved again would be necessary for retail in England.
https://www.hallowellco.com/proof_date_codes.htm



















































