1990 era Traxxas XL-1 electronic speed controller
That would be around 50 years ago now.
In those days a servo was an electric motor powered escapement. I had a rudder servo that drove a screwed rod with a travelling 'nut' which was for the rudder movement. This needed 2 switch controls one left and one right, the rudder would move end points and then the end microswitch would cut the current and when you released the Tx switch it would centralise the rudder again. Very sophisticated!
Back then for speed which was from dry batteries I had a sequenced pair of relays that would run the batteries in either parallel or series with stop in between. Still got them!
I never did use i/c always electric.
Roy