Lets tell a story.
In one of the trucks that I fitted the high speed transfer gears, the shaft out of the gearbox was butchered by previous owners. I ended up taking the gearbox apart to have the shaft spline reground. This is in a FG637, but to my knowledge the same gearbox is used in all the earlier range of Canters. Now out of the factory they are setup to be fitted with syncros in (first and reverse) that aren't fitted in there.You need to do reverse aswell to balance the shifting keys. The positions are there, all you need is syncro rings, springs, and shifting keys which I'm believing are the same ones as second/third. Now of course the previous owner could also have changed the internals of the gearbox.
This I will be finding out in the near future For my "Yes or No" Vehicle.
Dan.