First, I went looking for a picture of Ian's weird and wonderful gun to show a friend, what with the markings and such(since trying to show a video on a phone is a miserable pain). And, during said search, I found a picture of exactly such an obrez as mentioned, side by side with a Martini Henry it would have been made out of...so, of course, I can't find it now.

On the other hand, I did find THIS, which I found both interesting and at least tangentally connected, as it were. They're chambered for notably less punishing rounds than a .303 British, but still.

I also found THIS out of the British National Army Museum, which on the one hand is claimed to be exactly such a gun, that is a cut-down Martini Henry. On the OTHER hand, it is also noted to be made/used by "North West Frontier tribesmen"...which, if you were to take a closer look at the meaning of that sentence, means "from the region of Afghanistan/Pakistan"...anyone care to guess how well that matches up to "Khyber Pass"? Yeah. Today, it's even called "Khyber Pakhtunkhwa".

BUT, on the THIRD hand, and I'm starting to run out of hands here, the gun doesn't have much in the way of markings. Now, that would ordinarily sort of indicate "not actually a rifle", there aren't any royal crown, no V.R., no "Enfield". But, there's also none of the MAD amount of wild-ass stampings and engravings that Khyber Pass-made guns tend to be covered in. This, along with the fact that it's in the actual army museum would, to me, tend to indicate that it is what it purports to be.

...!
Stoke Mandeville, Esq & The Victorian Ballsmiths
"Nobody Want Verdigris-Covered Balls!"