It sounds just about right for a shifter detent. If the box was opened on the top, the shifter detent could have fallen in. I had this happen on one transmission that I did a post-mortem on, which is why I know that this can happen very easily.

The sticky first gear thing sounds like a different issue, though. The shift gate probably needs to be adjusted slightly. There are probably a few places where a ball bearing could have come from. Who knows if the thing has lived in there since the trans was built.

That doesn't sound very good.
The detents are captive within the top case above the shift rail, so I would doubt that is what it is. Also the size suggests it is the rear main or front main bearing.
The most likely is that when it was once opened, a bearing was dropped into the bottom of the housing and never retrieved. I would think that the trans would be very problematic if one bearing was not in it's place.
If it were me, I would drop the tranny and look. You will need to take the trans out and look from above. If the rear bearings were replaced this may be what happened.
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'89 245 Sportwagon, '04 V70 2.5T Sportwagon