Both the ball bearing type and the scissor type side covers were used in that era. I believe they finally settled on the ball type in later years. It could have a weak spring as you noted it was pushed inside the shift fork. I would replace it. It should shift smoothly but you should be able to feel the detent. As we discussed there isn't much that holds them into gear so any weak link is a problem. It shouldn't pop out of gear this soon though, it usually happens on a long run when it is warmed up.
You can pull the clutch gear out without disturbing anything. Remove the side cover and the bearing retainer and pull it out.
If some of the needle bearings dislodge just pick them up with a magnet.
Fred
