John, Thanks for that. It explains a perfectly normal Long Branch No.4Mk.1* that I have here with a hand stamped serial number from a 77L, (1944 dated), rifle on the butt socket. The receiver is a Parkerized CAL/LB replacement marked C No.4Mk.1* and dated 1956. It's also one of my favorite rifles to shoot.
I also have a 92L Long Branch rifle that's had it's receiver replaced with one from a 1943 Maltby No.4Mk.1! The Maltby serial number has been linished and the Long Branch serial number stamped in it's place. It's a real oddball indeed but feels right as rain. The kind of thing that always stirs my curiosity. Brian