yep DThomas
now that i re-read it... doh!

in the models that had non-scr controlled hydraulics the speed was controlled by the control valve. On those units the hydraulic motor would spin up to full speed or a pre-determined speed with no variation on the motor itself. The lift speed was controlled by a variable flow valve that changed as you moved the hydraulic control stick/lever.

Now on some of those units i remember seeing a potentiometer on the hydraulic control button or **** which controlled a variable valve on the manifold block but even on those the motor still had a preset speed it ran at which was generally full rated rpm for that motor.

Now if I'm not mistaken the trucks that did have the MIB did control the speed of the motor for the drive and hydraulic system and also the auxillary speeds for the accessory functions , ie: reach, tilt, side shift and even steering but those speeds were also preset speeds according to the auxillary motor speed setting.
As far as the AC trucks those were the first ones that came out with an actual controller that controlled the hydraulic motor speed to regulate lifting and function speeds via the motor instead of the control valve alone.

At least that's the way i remember it, I'd have to dig up my old manuals and bone up a bit to be 100% positive, it has been quite a few years since i dealt with yale trucks though so my memory isn't what it used to be ;o)