There seems to be a lot of confusion about this.
Here is the deal:
Old System New System
Option 11 CS1000M (Cabinet or Chassis)
Option 61c CS1000M S/G (Single Group)
Option 81c CS1000M M/G (Multi Group)
The CS1000s (small system), consists of a small Call Server Module and 1 or more Media Gateways. The system uses the SSC as the main processor. The Call Server module is analogous to the main cabinet of an option 11, but has no card slots - you lose card slots 1-10. Each of the 4 supported Media Gateways are analogous to an option 11 expansion cabinet.
The CS1000e (enterprise) is the new name for everything starting with release 5, if you have any IP in the system.
Also starting with Release 5, the CS1000t (trunking) system gets rolled back into the CS1000e, they no longer need to be separate systems. Although you can leave them separate if you want.