Sure there are 3 models of the Studio Color: "M", "S", "575".
"M" - was the first, and stands for Magentic ballast....these units are a bit heavier than the newer ones, and only run off 208v or higher....you can identify them phsyically by the fact that there is no heat sink on the top-box, and the arms of the head are more angular and have rounded "circles" at the ends on the side.
"S" - was second, but stands for Switching ballast....these are nearly identical to the newest 575s internally, but you can tell them by the fact that they do have the heat sink on the top box, but the older style arms like the "M"
"575" - is the current production model and looks nearly identical to the "S", but has the same arms as a Studio Spot-575....the release of the Studio Spot prompted this change to make more parts compatible between the fixtures.
The "S" was also the first to use position sensors on pan/tilt to recover from being bumped. The "M" does not, and if it gets bumped, it's rehome time . . . . If I recall correctly, the logic board uses the sensor input and lamp supply feedback to determine what version it's on, since the "M" and "S" logic cards and software are identical.
