There are no production numbers for GM for the 50's. However they were not as plentiful as conventional cabs. The fact that they were 5 window does not make them especially rare. cabovers were ordered to more easily maneuver in tight situations and 5 windows provided better visibility. There is no real established market on larger trucks because there is not much demand for them. They are not very practical to cruise in and don't fit in most people's garage. I guess it all boils down to if you would rather have the truck sitting there or rather have cash in hand.