Okay omfg, I don't know how I missed this post last week, but since I am looking at them myself this week, I found "part" of an answer for ya. Here is what I see in the Agility 50 manual:
• Be careful not to lose the steel balls (26
on top race and 29 on bottom race).
Does not mention size or anything so, hopefully, when the need arises, at least one of the original ones will still be in good enough condition for a match up at a local store. Are you replacing yours?