Truck mount carpet cleaner: where did the term come from?

Truck mount carpet cleaner :

Though introduced in the late 1960’s and early 1970’s, the truckmount became the dominant piece of equipment for the carpet cleaning professional in the 1980’s. Like any new revolutionary change, there were those who were resistant to change and slow to adapt. It might be hard for many cleaners to understand now, but there was a long time when the hardest part of selling a truckmount was convincing the carpet cleaner the increased productivity of the truckmount was worth the extra investment required.