For the record:
The very first D2s (from 1974) were fitted with a 2-prong receptacle for the power nozzle; from 1975 onward they used 3-prong receptacles, but by the end of production in 1979, some of the 1975-79 D2s were converted to 2-prong receptacles.
The first D3s (which replaced the D2 in April 1980) still used the 3-wire receptacles for the power cords, but the D3C switched to a 2-wire receptacle.
~Ben