Prologue 21 SKC-1000
The Prologue 21 SKC-1000 was the original release version of the Prologue lineup. These models were based on the VA0 family of Sega Saturn, with some changes to headers on the board for use internally in the machines, instead of having connectors populated for interface with controllers and cabling. These revisions even include a VCD card connector and usually a CR2032 coin cell battery holder for retaining game saves, although these are near inaccessible in practice. Approximately 7-8,000 produced.
Super Prologue 21 SKC-1000
The Prologue 21 was upgraded to Super Prologue 21 in mid-1995 with the addition of the Harmologue board inside. This enabled greater range of voice detection and modification than units without included. Most units by this point omit the CR2032 battery holder, as it would have required extensive disassembly to change. Approximately 9-10,000 produced.
Super Prologue 21 SKC-1000C
With the success of the Saturn and Prologue lines, Sega saw fit to redesign the internal boards making up the machine. Extreme consolidation took place, and resulted in a much more reliable and efficient layout internally. Rear I/O is identical, but is rearranged. Approximately 22-23,000 produced.
Limited Prologue 21 SKC-1000C
The Limited Prologue 21 has no information available I can find regarding differences from the Super Prologue 21. Despite owning one of the only two units I have seen, the only differences I can come up with are branding. Instead of Sega, the unit is branded as SGM, and has different overlay stickers that omit mention of the Harmologue function and Saturn logo from the front, though both are present. Functionally, it is identical to the Super Prologue 21, and even boots with a Super Prologue splash screen. I am unsure if this is as intended or the hard drive image is different than as shipped. Unknown production, though I assume 500-1,000 produced.
SEGA Prologue 21 