Ok I Might be wrong here but the ISM and the ISX are the new line of cummins engines. The ISM is replacing the M-11 and the ISX the N-14red top. The ISM is the smaller displacment 300-400 HP engines and the ISX the bigger 600+ HP capable engine. I do believe the ISX is a twin spool engine but not sure about the ISM.
The ISM is the upgraded M-11 model, which owes it's heritage to the older still L-10 series. I believe the top power rating for road use is right at 430 hp with a torque rating somewhere well into the 1400's, but don;t hold my feet to the fire on that. Very decent engine, good power and lighter than the ISX/855 models by about 800 pounds. If you need lighter weight, they are a pretty good choice, with all the bugs worked out ove the years.r