Yes the 21 shock is taller, i got hold of some 21 plates but they made the ride height taller than what i have there. You cant see in that pic but the plates are engraved "NSR" and to my knowledge the conversion was done in japan so i assume the plates may have been a off the shelf kit produced there.

Problem is my 18 R model shock is like a pogo stick and in my opinion its a waste of money rebuilding it, id settle for a 18 SP shock and have that rebuilt and fettled but yet to find one. im not keen on messing with linkage dimensions to achieve the correct ride height of a 21 shock as that alters more than just height and i'll be into unknown territory with that.

Ride height is currently fine on mine so if you would like the dimensions of these plates just shout