I hate gothic letters :tinysmile_fatgrin_t
Yes, you are right (for low-level bombing)
It's quite possible. SC mean "Sprengbombe, Cylindrisch" or Cylinder - explosive-filled bomb with cylinder shell. Most of previously used German bombs were streamlined (spindly-formed). AFAIK thick-walled bomb were introduced in 1938 or '39 and, as it was 50 kg bomb, they must change marking for distinguish thin-walled and thick-walled model. So they introduced SD - Sprengbombe, Dickwandig (explosive bomb, thick-walled).
According to 1940 date of this manual, there's an information on the last page that it substitute previous edition from 1935.