Watson was by far the largest manufacturing concern making auxiliary transmissions until spicer bought them out. GM dealers used them en mas. Until the mid 60's. My family probably sold 30,000 of them over 30 years. But that was over 6 gmc dealerships. I have sales docs for about 5k of them. Since I have about 70k of sales docs on gmc trucks sold from 1951- 1977. My info tells me most were sold in the mid fifties to mid sixties. Pretty common up sell. Dealer installed.