big chance it's the "Jesus screw" that's missing.
ok, now,
1. the x11-spur fits the X10 as well (exactly the same dimensions AFAIK only a different material.
2. that X11 (and x10)-spur is indeed the usual 48 pitch.
to check:
3. check the pinion: is it alright? (no broken teeth, too much wear?)
4. check the spacing between pinion and spur: you should have just a veeery litle play. VERY IMPORTANT.
5. Motor should be secured very tight with the motor-screw (I replaced that screw by a stronger and longer stainless screw so that I could apply more force to it, without the risk of damaging it.
6. Check Jesus-screw(
*)
7. Check the Maria-screw (
**)
8. Check all 4 screws on top of rear diff-house (
***)
9. check that there is still some slipper-action, in other words: the slipper should not be super tight.
The motor is usually strong enough to strip the spur when there is no slipper-action and the car is stuck against something.
I once had the same problem when I didn't check these Rear Gearbox Top-screws after a diff-job.
The gearbox then slightly lifted and by this allowing the propshaft to move more ... making the distance between spur and pinion greater then it should have been ...
It only took one bump where the car was blocked by another car, I hit the throttle by mistake ... and I saw white X11-spur-teeth flying around.
Learned a lesson once more
I'm curious what you might diagnose as your spur-problem.