I believe I am correct on this. If nitro proofed at Birmingham it should have a crown over the letters BNP (Birming Nitro Proof). if proofed in London it should have a raised, bent arm holding a Scepter over the letters NP (Nitro Proof). If not Nitro proofed from Birming it should have the crown over BP (Birming Proof) which was black powder proof.
I did not find a separate mark for a London proofed gun so believe it simply goes back to the original London Black Powder mark of a Rampant >Lion over an intertwined GP (Gunmakers Proof), without the Nitro Proof mark. At some point in time, but do not have the dates for this off-hand the words Not for Nitro or Black Powder Only were added.
