As I pointed out in the original post, I have seen these nuts get loose numerous times. They are not supposed to be reused. I have never seen one come so far apart as to allow separation. In earlier transmissions and maybe in yours the speedo function stops or swings madly as the gear isn't splined to the shaft. It depends on the tightness of the nut to spin.
Before it came apart there should have been considerable vibration.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
[This message has been edited by stevebfl (edited 08-10-2000).]