the gearbox on jato has started to jump out of third and crunches into fourth.now the question of what other boxs will fit ,it is 1725 with o/d or is this a common selector hub fault or does the end nut come undone causing freeplay.
Gearboxes are usually very strong but as with anything else they can wear. I did have the end nut come loose but it didn't have the same effect as yours, probably is the selector worn but try simple things like checking oil level first, it should be filled with good old 20/50 engine oil. The same box was fitted to a lot of Rootes group cars Rapier, Sceptre, Super Minx etc - basically every medium saloon they made in the 60's and the later Arrows Series but with a different bellhousing and input shaft. There a two set ups for the input and output shafts, early cars had less splines than the later ones so different clutch plate and prop - most Commers have the finer splines.
there is play in the bushes on the gear change linkage witch has always been there.it started to happen on the way back from scotland if i go into fourth at about 40mph it does not crunch