That is just an insane amount of electronics in that base! I figured for sure it was just a power+datawire adapter. I figured it would just have a bunch of wire, maybe one small circuit board with one or two small power regulation chips.
Why the H#!! did the even bother making the back removable, if it is so absolutely/positively required for use? Maybe they were going to make a second smaller base? Maybe they were going to modify the SCSI ports on some Macs to interface directly with the 'spinner' unit, bypassing all normal interface standards? (AKA: Interface direct, not even via the SCSI protocol.)
Now that I think about it, that would have rocked for use with PowerBooks. Cable going directly from the micro-SCSI on the PB to the 'not-SCSI' DB-25 on the PowerCD, and have the PowerBook do all the actual 'drive control' directly (IWM-style) as well as supply power. That would have rocked!