This wasn't to be a restoration thread but a little update, assembled all the parts to test it and as it's under lever not break barrel I overlooked finding the fact that there's a pellet already jammed in the barrel and so have added another when test firing, silly mistake.
Jammed solid now so I hope it's just two but obviously could be more. No bother, will overcome this.

I'll take reference from both the Knibbs and John's excellent BSA books but would like to test knowledge here anyway by asking about the fact that these two differ in length by 2 inches. Both are serials starting S and are 200 apart as stated above but the new one has a shorter cylinder which I only noticed when I eventually put them side by side, 77xxx is 45 1/2 inches (straight hand stock) versus 43 1/2 inches for 75xxx (pistol grip with heel stock) so suspect I have a standard and a light pattern? Asking because my 1920's light pattern L15xxx is 40 inches so is 3 inches shorter again, all 3 stocks are 14 1/4 inch.