Yup that's exactly what I've done mate, a dab of locktite just for good measure. Got a good tighten on the thread, to the point when I nip it up the gear will shift. I ended up with 3 spring washers and the nut just flush with end of the thread.
looks to be some decent thread there still if you can stack a couple of good washers on the outside of the selector arm and then fit a new nut where the ok threads are.
The BW40 selector shaft seal, is same as the BTR 4speed.
It's just a small lock nut, with a flat washer behind it.
The shift lever is held in position, via the selector shaft having two flat sides.