Apologies for yet another post. Below is an enhanced view of the proofs.
A few observations from someone who is not an expert:
1) The upper left mark of an arm w/ sword and "NP" is a London mark used post 1904 for definitive nitro proof.
2) The italics "R" under the crown at right (above the CHOKE mark) shows reproof at London, in use post 1925
3) The BNP under crown is a Birmingham proof used post 1954.
Note that this William Ford serial number is 2197. My Ford has a 14,000 serial, and was dated to 1929. So the gun I looked at yesterday is presumably much older. To recap, barrels are 26" and HEAVY. Total gun weight is 9.6 lbs. Front trigger is articulated. Address, if I recall correctly, is not the Saint Mary's Row address on my Ford but different.
So is it possible that this started as a chamberless 12 (12 gauge shells with a 10 gauge bore and no forcing cones) and was reproved, possibly for nitro or ? in London, receiving the 12c proof (shells of 3"), and then later re-chambered and re-proved for 10 gauge 2 7/8" shells?