Hi Dan, they do yes, hence my Rock Rider comment...although they are almost identical, the Pirelli feel flimsy by comparison, and you don't get the uneven tread wear with the RR. Each to their own.
Try the Mitas MC21 ROCKRIDER, cheap R1150 great Tyre similar to a Pirelli 21, but a 4/3 split on knobbles instead of 2/3 you don't get that flattening out between treads. ( my cents worth) can only comment front Tyre.
Run the MT21 on my 990,s (front) but only when the Mitas Mc23 Rock Rider not available. the Mitas is pretty much same tread pattern, and grip off road, cheaper R1050 fitted and a stronger Tyre. (my view)