Bennett identified their pumps with a series/specifician number system. The first number would be the series number, or general description of the pump, the second numbers would be assigned to describe features of the pump. Example: 646, 656, 666 all were the same series but has different features to suit gas station owners tastes in features. I believe this began with the 500 series of pumps, then the 600/700 series, 900/1000 series then 2000, 3000, and I believe 4000 series at the end. There was no 800 series. The 600 and 700 series were mechanically identical, but the 700s were short while the 600s were tall. The two series share many parts. The 900s were basically mechanically identical to the 700s but were cosmeticly updated to fit style trends. The 1000 series looked similar to the 900s and shared many parts, but used an automatic resetting computer, new florescent lights, and relocated the sight glass to the side of the pump.
The 2000 series was introduced at the tail end of the 50s and is a radical departure from the previous models in style, as boxy was seen as a new and different style. It used most of the internal parts from the 1000s, and was really only different in style. Mechanically, there would be few changes from the late 40s to the late 1980s when gas pumps with pumping units inside them were fazed out in favor of more efficient "dispenser" technology.
