2640 with no high range

I was discussing a transmission problem this morning concerning a 2640 that doesn't move in high range. It's not a reverser
transmission, it's manually shifted into reverse. It does have the high lo hydraulic shift unit, but that's not where the
problem is. It has the two shifters, the right one for the four speeds and the left one for low and reverse, high and park. This
is the high range that's not working.
I've been studying the I&T today, which is better than nothing, but a bit vague on explanations. I see a breakdown on the four
shift shafts and four shift forks. The one I'm concerned with first is labeled "shift shaft, high range". The other three each
control two positions(1st - 2nd, 3rd - 4th, lo - rev). I see no mention of how park is engaged. I first thought it was with this same
hi range shaft/fork, but now I don't see how that can be. I guess park is engaged with the left side shifter but not with
this fork? I didn't ask specifically if park still works, but it wasn't mentioned.
If the shift into high range "feels" as it should(detents seem solid enough), then maybe the lack of engagement could be an
issue with the fork being broken or maybe just come loose on the shaft?

My guess is tab on shift rail is broken similar to photo. I've witnessed these broken pieces welded back on with shift rail still in tractor.

After more thought set screw that tightens fork to shift rail could be loose. One would need to remove rockshaft housing for visual inspection.

I think an issue with the fork is more likely because the rail would need to be moving in order to feel the detents.