Probably a 3/4 syncho and the meshing gears, about £200. the 3/4 synchor has a very slight angle (a degree or two) on the dogs to hold it in gear when the torque is on, once they wear parallel it will come ouf of gear. The detents will help but once the angle wears the force will eventually overcome it. Its a simple job - box out, strip and replace those parts and its worth budgeting for as you can do the seals and clutch and release bearing at the same time and odds are some thing else will need doing, but its a hard (physical) weeks work. I think you have to take the view that whether it jumps out of gear or not, if the box has not been stripped for 10 or more years it probably will need to be soon, even if its just the seals. Get some money off and go for it. Take the view that every series you look at will end up needing a box or engine strip, but some will be more honest and reflect this in the price (lower) while others will not.