@saper @phooky That's as likely as anything I could come up with. It's also possible it's an analogous product for an IBM competitor's systems, since it physically doesn't resemble IBM's SLT products.

Searching on the item number (1488503) doesn't bring up anything useful, which is kind of unusual for old mainframe hardware.

@saper @ardgedee Wow, that seems very likely! I seem to remember having some cards from the a similar haul that looked similar, and the backplane connectors are a very good match. Thanks!

@saper @ardgedee It's very similar to this system/370 backplane-- I think you've nailed it!
http://moca.ncl.ac.uk/circuitry/fullsize/200433b.jpg

@phooky @ardgedee your backplane looks way smaller, but there is something similar, yes!

I think I got to understand the origin of the word #mainframe itself. They were delivered as empty cabinets (frames) that had to be filled with those backplanes and cards... As StackExchange suggests it goes back to big cabling frames in the telephone exchanges.

@saper @ardgedee I spent an hour last night on bitsavers looking at old system 370 part catalogs, and you're right: they're basically really, really complicated Ikea shelving that you'd hang a computer inside!

@phooky @ardgedee reminds me of http://helmer.sfe.se/

I think I am going to fire up my VM/370 on Hercules out of nostalgia!