Well, my F150 only has rear axle ABS and it is single channel so there is just one speed sensor there.
Problem is that my '85 is the last of the 1st Gen cars that came with it in the US... and if they are proprietary to 1st Gen cars I am going to be up creek as I am pretty sure only the only 1st Gen cars here in the US to receive them were in 1985. Most of the gasoline ones are long gone... the 500SEL and 380SE due to our favorite Cash 4 Clunkers program.
I am pretty sure my fronts are clean and it would only make sense the rear is single channel ABS, so in theory the car should have just three sensors. I am pretty sure the cars with ASD? or ASR received four channel (four sensor) ABS.