Aha so that answers my question about why they haven't at least switched to USB thumb drives for their sneakernet. These aren't PCs. Probably minicomputers such as a TI-990 or similar. They don't have USB ports.
That said the 3.5" drives were actually electrically compatible with the double density 8" SA-800 drives.
If anyone wants to see the guts of an 8-inch drive and how complex it is check out this video from Adrian's Digital Basement.
Can't believe how interesting watching that was, thanks. I learned about GreaseWeazle which I might use one day to backup some of my old copy protected disks.