Late 1995, the Prologue lineup launched a new, more refined Commander unit redesignated as SKC-1000C. The original SKC-1000 released with little consolidation done to the boards inside, to the extent that the main motherboard was in essence a launch Japanese saturn with headers populated where any external connectors would be. These even include two power supplies internally, one for the Saturn board, and a second for everything else. The SKC-1000C consolidates both of these together, and most boards onto one main board designed specifically for the unit.
I have seen a couple examples of a non-Super or Limited SKC-1000C at points. I assume this was a less expensive basic Prologue 21 without the Harmologue board produced after the SKC-1000C redesign. I have not obtained one or been able to otherwise confirm as of yet though.
SEGA Prologue 21 