My Dec 1950 XK120 660405 still has its original gearbox - JH 5207.
I have a spare gearbox whose number is JH 5262 which I am about to have rebuilt, to be put into the car until such time as I get my original rebuilt. It would be interesting to know which chassis number received gearbox JH 5262 originally. Does anyone have such information, I wonder?
It could also have come from a Mark V. They are the same other than the bell housing and shift lever.
I was told by the seller that it had come from his father-in-law’s XK120. A bit of research on xkdata shows other numbers that are close: JH 5259 - 660423; JH 5260 - 660444; JH 5263 - 660434. All Dec 1950.
You could take the best of both transmissions for the rebuild and keep the original housing preserving your numbers matching status. That is the way we used to do it in the old days when replacements parts were not available.
It will be good to inspect the JH5207 case carefully for sometime-seen cracks when dismantling for rebuild.
Had a pretty good look at the gearbox when I was doing the floors. Took the top off, too. I didn’t notice anything amiss then - hopefully all will be OK. It functions well enough, just rather noisy on the bearings.