By 1990 it would have been a desktop S/36 which sat on or under your desktop. The first version was the shape of a washing machine but about 6x bigger by volume.
It was also incredibly slow. I later came across an insurance company that ran its business on a S/36 emulated on a 386. The emulation was faster than the original machine.
From memory I don't think the architecture had much in common with S/360. Quite simple I think but a lot of the internal state of the processor exposed to the programmer. Might do a blog post on it at some point!
Edit; Do let me know if a blog / newsletter post would be of interest - details in my bio. Thanks!