My No1 MkVI is one of the ones re-configured to No4. I bought this 21 years ago, and it had a No 4 buttstock on it (no stock marking disc). I bought an NOS stock and forstner bit to fit a marking disc, purely for aesthetics. The original cypher and designation had been scrubbed off, and a five digit serial number stamped in its place. It is 23560A. This number is stamped on the barrel and bolt. Interestingly, the barrel has a 41 date on the opposite side of the barrel serial number. A large BB 22 is stamped underneath the barrel. Majority of parts are EFD marked, with a mix of S126 and N79 parts thrown in. When I first got it, it was packed with cosmolinebeneath the wood. No import marks and no BNP proof either.
Sorry for the poor quality pics. I had some close ups, but they were too big to upload. I'll play with it more tomorrow.