Jack,
I certainly do remember them. I used a Kelsey-Hayes hydraulic-electric controller from 1972 when I bought my first travel trailer through the '70s. I was again using one in the late '90s on a '94 Ford F-250 pulling a triple axle Airstream until I bought my '01 Ram. I installed and used it briefly on my '01 Dodge but the higher brake line pressure of the four wheel disc brakes and large calipers of the Dodge was tough on the K-H controller. Normal braking would slam the potentiometer lever so hard it would have destroyed the controller.
I gave it to a former TDR member for use on his '01 Dodge dually with front disc/rear drum brakes. He has used it for a couple of years now.
The K-H controller definitely can be used with dual master cylinders, ABS brakes, even front disc/rear drum brake systems but could not tolerate a four wheel disc brake dually with big brakes.