(As a cross-check, I did try to see how easy it was to put the 320 end
piece back
on the 320 and, given my general competence level, it wouldn't go on
either!

To put each end piece back is a bit tricky but not difficult. Just try to
put it back GRASPING the whole cartridge with one hand (wearing a glove for
not damaging the opc drum) and the end plate with THE OTHER HAND. Then try
to put it back with no much force, just play pushing at different places
(and moving it in all directions, left-right, up-down). You won't understand
when it will be back!!

Another trick is, while you have grasped the cartridge, grasp with a bit
force (the two sections of the cartridge will move together a few
mili-meters, there are two springs for this reason).