They look like Brit Proofs. There maybe something on the bolt too. The Carbine Club would be interested to hear about your one as they are collecting data on Brit Proofs. Mike.
BNP stands for Birmingham Nitro Proof. The mark means the carbine came into the hands of a Britisharms dealer at some point in time and the proof test and mark were required by British law before the gun could be sold.
Yes, the bolt is stamped. It is the only part out of place on this one. it has a N 6 bolt. The bolt looks new right down to the finish on the face. There is no punch mark on the bolt. The BNP Underwood I got also has a very good NPM bolt stamped BNP but no punch mark.
I will send in the data sheets as soon as I get my copier working.
John