Ball bearings falling out

Don't think there's any balls that small inside the trans. There's roller bearings inside the countershaft, but they're not balls. Same in the input shaft. The thrust bearings are flat spacers. The ball detents in the cover are much larger.

I can't imagine anything could get out of the front bearing retainer. The small hole in the front bearing retainer is just a vent, if we're talking about the same thing.