wrong here is a pic of my 1950 f5
anything over the f3 was a cab over like mine

 

The reason the "150" designation was started was to distinguish the heavy duty half-ton truck from the light duty "100" series. This was due to the emmissions regulations imposed on cars and light duty trucks in the mid 1970's. At the time (I think it started in 1976), all trucks with a GVWR of 5000lbs or less had to meet the same emission standards that cars had to meet. This meant that they had to have catalytic converters, smog pumps, evaporative emissions controls, etc. Ford and Chevy began selling heavy duty versions of their half-ton trucks (chevy's was called the "heavy half") with a GVWR of 5250lbs so that they could offer a half-ton truck to their customers without all of the emissions crap on it. By 1979, however, the government cracked down on this "loophole" and bumped the GVWR requirement up so that even the heavy half trucks were included (and some 3/4 tons). I believe that the cutoff for emissions equipment from then until somewhere in the mid-late '80s was 8000lbs. Anyway, since there was no distinction between a F-100 and an F150 truck in 1979 except for the slightly higher payload of the 150, the 100 name was slowly phased out (I think after the '81 model year) and replaced soley by the "150" designation.