Dear Campero,
The BNP mark signifies Birmingham Nitro Proof, so no worries there.
The viewers mark to the top left of the bore size marking is a pair of crossed sceptres (the original Birmingham proof mark from 1814, when they adopted what had been the private proof mark of the well known Birmingham maker Ketland). They look more like crossed daggers.
The 1 in the bottom quadrant indicates that it was viewed after proof by the senior examiner.
The letter in the left hand quadrant shows the year it was re-proofed, A for 1950 through to Z in 1974 but they skipped I for some (or no?) reason.
On your gun the year letter is not very clear, and it looks like the punch was not held very straight. It looks like I, which was not used, but could be L for 1960?
There was a double Wiggan and Elliott shotgun in the last Southams auction sale. Unfortunately they delete their online catalogue after the sale ends but I am pretty sure that it was a 24 bore, and that there was also a lot of 100? 24 bore cases or cartridges. If it didn’t sell it will no doubt appear in their sale in March. If you contact them guns@southamsauction.co.uk I am sure that they will be happy to send you a printed copy of the December 2020 catalogue if they have any left.
