Ben Heck recently had a chance to inspect and repair a rare Sega Pluto Prototype. It appears to have mostly standard Sega Saturn internals, with a few extras such as a modem and a hard drive.
There’s still many unanswered questions though: What was the hard drive used for? Did it have the extra ram integrated to play RAM cart games, or was that still a required external option? Did it come with the hardware for VCD movies built in? Will it work with the mCable (;p)?
A video with much more questions than answers, but if you’re a fan of the Sega Saturn, try to concentrate on the hardware aspects of the video, for a glimpse of a rare console revision!