I asked a friend who knows:
Regarding the 210 & 210M machines, many of the parts are the same, as far as heaters and sponge pads go. The really old 210 models had a different type of thermostat, however the current will fit them. However, the really old units had larger lights and a different switch that aren’t available anymore. There may be a few minor mechanical differences in the very old versus the more current models, such as in the lever arms and toggle plates, which aren’t available for the very old. However, I don’t see too many of the really old relics anymore, and there hasn’t been a whole lot of changes in the machines over the last 30+ years. So just depends on how old the unit is.