seaguy said:
Anyone know what these stand for?
"20" Is the model code number for the Atlas, which the Commando engine series was a continuation of, so "20M3" is supposed to mean Model 20 (Atlas) Mark 3.
As chris plant has said, the 'S' was added to denote camshaft driven points, because 20M3 models had a separate points housing in the old 'magneto' position behind the barrels driven by a second chain.
The letter 'S' was probably chosen as the 750S Commando models were the first to have camshaft driven points.