very common situation on these old 'bolts, partly caused by sloppy shift lever movment, always follow the "H", don't try to "cut corners" .... the spring ends keep the spring in place, but maybe what you're seeing is the spring broke [yes, most vendors offer them] - you can see one end [straight]
in this pic and the other end is a hook
seen here .... the most important part is that the forks be perfectly aligned when in neutral and that the inside corners of them aren't real rounded off - and while you're in there clean out old grease and re-grease it [don't pack it full] - you may also need to adjust the arms outside the box that go down to the trans, they can slip over time if not tight enough, and as Cletis mentions make sure the bushings there and at the trans are good
Bill