The S has a solid state lamp power supply, the M a magnetic (hence M and S . . . ). The S also includes position encoders on for pan and tilt, which are not in the M. Logic, DMX, and motor power supply components are the same . . . the M was a product put out due to problems with the solid state lamp supply when Studio Color was supposed to ship, so HES put in magnetic ballasts (creating the "M") in order to start shipping . . .
