Hi John. I had an improved Mod. D about twenty years ago that had the same tap set-up, so that doesn't phase me or suggest it is anything other than original to me, and also, and as stated earlier, as the barrel sports the BSA piled arms logo between the rear sight and the loading port, it is doubtful that it is from an LJ?
Further, as the trigger action assembly and butt are clearly BSA, and as the wording on the cylinder is as I have learned, the same as seen on many early BSAs from the period when LJ still owned the patient (according to the wibblywobblyinterweb), I think that since the alarm was raised by a well wisher that this gun may be a bit of a bitsa, that I have been chasing ghosts which simply don't exist?