Quick Theory and Speculation
The Pluto is largely the same as the stock Saturn with some small changes. Besides ASIC revisions, alternate parts, and layout changes what makes the Pluto unique is the extension of the cartridge slot to an external bus for the modem and hard drive. The cartridge-to-external-bus is buffered with some bus transceivers which appear to be controlled by IC50.
IC50 looks like it might also act as a middleman between the cartridge and external bus. IC50 is probably some Altera or Xilinx FPGA, or maybe Lattice based on the part choice for IC52.
Considering that the Modem and HDD PCBs don't appear to be from SOJ or Cross Products, the use of a Seagate HDD instead of a Hitachi one, and that both Pluto's were discovered in the US, my guess is that the Pluto was made by SOA to sell the idea of including a modem and hard drive in the Saturn to SOJ. Or maybe as a concept idea for the Dreamcast? I don't remember all the rumors and stuff from this time period. But going through all the trouble and engineering effort to make it look like a commercial product is a bit much for a simple proof of concept.
|Item||Sega No.||Manufacturer||Manufacturer No.||Variant No.||Details|
|PC/IC BD PLUTO MAIN||837-12472||171-7211B||C SEGA 1996|
Disc Drive looks like a variant of this one https://commons.wikimedia.org/wiki/File:Sega-Saturn-Laser-Assembly-V2-a.jpg
Modem and HDD PCBs not "MADE IN JAPAN". The "part number"s do not fit any Sega or Cross Products numbering scheme, they're more like file names.
Modem and HDD connect to MOTHER which connects to IC50 and the cartridge slot. Bunch of LS245 bus transceivers probably for buffering.