Hi  Fifty years ago speed shifting would cause this. Would replace second speed gear would be good for a while. Sync teeth would get round off, pop out gear.

get a short shaft to slip into the cluster as you extract the 1 it came without must be the length of the cluster and washers now with cluster resting on the bottom. this makes the oh very easy as you don't have to chase the needle bearings. you need a new synk row and likely 2ndd  gear. most isues in thes trans are rushing the shift without releasing the clutch enough ,proper free play on the clutch throw out bearing is critical