A little bit of grease will hold them in place during assembly. The "inner ring" is a groove in the trunnion.
Nobody said it was a great design!<g> But many, many were sold, and many are still in use.
Bicycles (from earliest days to about 1975) used loose balls in their headset bearings (the steering tube) and crankset bearings. That give you the most balls possible, for a given outside radius. There's no space taken up by a ball retaining cage.
Charles, I remember as a little boy, going to take the bike of my father apart, and all the balls were jumping out, I couldn't find back all, and I didn't know the grease trick at that time... May be magnetising the balls yield a mounting method without grease required.
Yesterday there was a local hamfest, and the Surplus Radio Society, had a key collector, that dismounted the knob, turned it around and it shows "Japan", he told it was a cheap replica of the J38, fabricated in the period that Japan copied a lot of western designs.
Finally the conclusion for the topic starter may be, he doesn't need a ball bearing, but just a missing ball of the right size.