It's a target box for Saturn musicians. I don't know what features it has exactly, but it's equipped with MIDI ports. I believe you need a SCSI card (and most likely a specific SCSI card) to communicate with it, and without it or any software it would require some effort to do anything useful with it.
omg this thing is weird... the text says 'Saturn sound board', is this some beefed up debug version of the Saturns sound system? Then, why is it so big and does it have lots of ICs, a location marked VDP1 and something to put a CR2032 battery in? But as far as I can see it ain't a Saturn either as it lacks some of the chips... very weird.
I think the Sophia Systems boxes were made before Saturn hardware production had begun, thus the "prototype-look" they have. The hardware may also not be totally identical to the final hardware specification and there might of course also be some extra "developer" hardware (eg. full 8Mbit sound RAM, 18-bit DAC, extra debugging facilities).
(ps. it's surprising you've never seen one of those before, especially if you hang around at places like ASSEMbler's site.)
well I've seen the thing several times, I just never saw its insides or learned its functions... I assumed it connected to some other devbox, which in turn could connect to the Virtual CD system and the EVA board or what was it...