To be honest - no idea.
The nut on the drive shaft was mentioned as part of caster adjustment procedure here on the Forum and also in my BBB, which is for 280SL.
In any case - the idea is that subframe will move backwards or forwards while adjusting. The drive shaft must be able to change the length - by whatever means is there available.
After the caster adjustment I did the procedure of settling the drive shaft as if it was after removing/fitting it (letting the nuts and bolts just semi-lose, move the car back and forth, rock it a bit and tighten everything in the car standing on wheels.