I'll be damned, you know what's buried there:
It's a hard disk. An 8" CDC Sabre or Seagate equivalent IPI-2 disk.
I've got one of these (a Seagate ST81154K) in my 4D/380VGX "Predator" rack. This is what it looks like from the back side:
It occupies roughly half a 19" shelf. This is what it looks like from the front when I pull the shelf out of the 4D/380:
The front half of what you're seeing here is the dedicated power supply. The keypad can be used to manually power the disk up, change it's status (online/offline it) and manually toggle bad sectors in. I think it was optional because not all Sabres seem to have it. The printout in the transparent envelope contains the list of bad sectors from when it left the factory. The whole thing weighs roughly 30kg and the unformatted capacity is 1.1GB. Internally it stripes data over multiple heads/platters so it's streaming speed is better than it's SCSI brothers which didn't do that.
IPI-2 was the high-end solution back when SCSI was in it's infancy. Some of it's treats (like the dual ported ability) later found their way into Fibre Channel. Dual ported + daisy chaining means there are 4 connectors on the back side.
It's very hard to find useful information about IPI-2 so I'll share two more pictures of how to wire it.
On the bulkhead of the system I have a pair of female 3-row 50 pin female D-shell connectors. Some old SUN equipment used 3-row D shells for SCSI, afaik none of that is compatible but can make it hard to figure out what is what if you run into both in a single haul.
Here the male end of the AMP cable in connected to port 0:
Unlike SCSI cables, IPI-2 cables have a male and a female connector. The cable connects to the 2nd connector from above (J4). The IPI-2 terminator is on the topmost connector (J3) of the disk. The other two connectors are for an optional second host (the disk is dual ported).
Like SCSI, IPI-2 requires a host adapter. In the PowerSeries, that's a Xylogics SV7890 full size VME board. The PROMs of the PowerSeries recognize it and can boot from it. You can use fx to initialize it and scan for bad blocks. My 4D/380 boots and runs IRIX 4.05 from the IPI-2 disk, or IRIX 5.3 from a conventional SCSI disk hanging off the IO3.