quadra 5 was non adjustable---10's were adjustable but you needed a long allen wrench to reach inside to do it----21's adjusted with the plastic thumbscrew on top----21R was same as 21, just "slightly" more travel and a different color---all used single piece elastomers but 21R had the little elastomer pieces with plastic connectors to mix and match firmness by using different colored elastomers--longer one piece elastomers came in different colors, too (cold weather, warm weather, etc)---springs were avaiable (which i had a pair once) and of course the englund air carts worked too, which i still have a set NOS---i am NOT a rock shox expert, i just happened to own at least one of all those models at some point in time