"High Modulation" is correct; Schneider used that designation for enlarging lenses too. What distinguishes the SS-HM series is their 80-degree coverage. They correspond to the Rodenstock Apo-Sironar(-W) series, except that Rodenstock offered 150/210/300 while Schneider offered 120/150/210. In the shared focal lengths (150, 210), the Rodenstocks are much smaller and lighter. Optically, they're all fine lenses.

More info here:

http://www.schneideroptics.com/info/..._symmar_hm.pdf