Hello! I'm in the process of restoring all the old door hardware in my house. I have a problem with two knobs that are disjointed from their stem/base. Where ceramic ball meets the iron base of it. (not the spindle. I already took them off the spindles, that's not the issue).

Here are a few pictures:

I have more of the same knobs that are all good and and I poked a little inside with a screwdriver but can find no screw or anything like that. On the two knobs that need repair I can see they have another set of holes that gets hidden inside the ball itself, when properly assembled - I wonder if some kind of a little spring device or something similar is what is missing. If anyone is familiar with these, please advise. Can they be fixed somehow? Many thanks in advance.