Louis,
I have a Watson Bros 1910 375 similar to your '08. It has the same sight about an inch from the muzzle with British proofs only. It is marked "NP" under the cleaver but that is not circled and is marked "Cordite 43-270 MAX" and "375 EX". This probably muddies the water more than helps, but that's what I do best! BTW the brass shield between the swivel and toe is marked "G le B.S. from J. and A. Moat" if that means anything to someone I'd like to know.
Take Care.
GT