Pity, but par for the course!
Interesting that you found it on the end of the forend - it would make sense when rifles were being stored in vertical racks, and particularly when the reaming process was underway.
I haven't seen that before and I'm not sure if it is known elsewhere. The same font so apparently done at the same time?
Hesketh Prichard in a letter to his wife mentioned making three touch at 100 yards off the sling with a Mk.III. He called it "a fluke as no one can hold as close". We might beg to diffah!