You're right and someone please correct me if I'm wrong as I'm not a plumber. Just looked at some generic Home Depot ones in my basement and no markings. I believe sched-40 refers to the pressure rating which is determined by the thickness of the pipe or valve body and the depth the mating pipe seats for a good pressure joint. Sched-40 pipes seat about 1.25". The ball valves have about the same seating depth. I've also always seen only one variety at a HD or Lowes. There are better, more expensive valves available but I've never used them.
Hope this helps.
Al