[rfeenstra]

Are you sure that the flange at the end of the shaft isn't just completely worn off or bent over causing the indentation that looks like it should hold the ball bearing.?  If I remember correctly on my 37-116's and my 38-116, the flange that pushed against the 3 ball bearings was machined in and the purpose of the ball was a bearing point for the spring to push against to reduce wear and friction.  the picture you have of the AA5 shaft is the correct shape.  The 3 balls may have frozen in the shaft and when the fine shaft was turned, it caused a lot of wear, eventually wearing the flange completely away.

Here is a photo of one from a Scott 16 where the fine shaft was worn out.  I had to make a new one on a lathe.  It works the same way.  The spring pushed on a ball bearing which then pushed on the end of the fine shaft.  The 3 ball bearings ride in the groove and are pushed against the flange.  The outer housing (not shown) holds the ball bearings in place.  As Morzh said, a planetary arrangement.