This is a simple one. The Panzer II C gearbox comes in two parts - A and B. Part b is near the wall and is the basis for the entire gearbox. All shafts, gears and bearings mount to it. Gearbox A bolts to that part, as well.

The issue is the Gearbox A can be mounted before all of the shafts and gears are mounded. This is a problem for two reasons. Firstly, it entirely obscures the inner workings of the gearbox, making it hard to actually click on things. Secondly, it makes no sense that the player is able to mount shafts and gears inside an enclosed gearbox

The solution here would be to require that ALL internal gearbox components be mounted before Gearbox A can be mounted and the gearbox closed.