Been watching this thread. Several Zroners have offered good info. There is a good article on Selective Ride Control which can be found by
clicking here.
In summary...
To change a 90-91 to the 2nd design SRC, one needs: The 2nd design shocks, which will have the right gears on them, and the 2nd design controller. Note that you
cannot use the early shocks with the late controller simply by changing the gear on top. It's the bypass valve inside the shock which is, also, different.
In a nutshell, the major difference between the 1st design SRC (89-91) and the 2nd design (92-95) is the speed at which the bypass valves can change states. The actuators rotate at the same speed, but the travel though which the bypass valves move is greatly reduced. Thus, the speed at which the shocks can change states increases. That makes the system more responsive.
Hope that helps.