I think thru most of the '60's into the '70's. Do a search on Saginaw PS Pumps - you will get lots of hits. Watch the model # as they were also used on a few other non-GM vehicles. Not sure, but they may even have been factory stock on the 283 & 327 motors.

1940 Chevy KC 1/2-Ton
In the Stovebolt Gallery