I have the rear end on my 2001 929 stripped down, giving it a once-over and a good clean-up, and re-lube as necessary.
Dropped the Shock Plates, and I think they were fitted incorrectly.
This is how they were arranged . . .


I've done some searching, and it seems they can be rotated/flipped to alter the Ride Height?
That's possibly what's been done here, as it currently has a fairly 'tall' Tyre on the rear.
I'm also thinking this is why the rear is overly 'stiff', even with the Shock Pre-Load on the lowest setting.

Anyone know the specs of the standard Spring?
Mine is White.
166mm free-length
12.58mm dia Wire
7 Coils
55mm I/D

Thanks.