[Doug in Maryland]

You are right: my experience with one of my AAs is that the teeth on the input shaft and the internal teeth on the 3rd/4th gear were worn on a bit of an angle from having been “thrown” out of 4th gear without the clutch which led to it jumping out of high gear. Replacing these solved the problem. However, if your friend’s gears are in good shape, I would think that as you said, the next most likely culprit is the output shaft’s bearing retainer is not properly installed or is simply missing altogether. I’m not sure about whether the small coupling shaft cup would fit inside the larger retainer. If your friend’s retainer is missing, I’m pretty sure I have extras lying around and would be happy to send one. Might take a few days from the states but happy to look if you’d like.