Interesting on extenders from Poland. I'd love to order some PCBs, if I could find a few dozen other interested people, from the gerber files on Github. I have plenty of receivers, and I think 2 or 3 IBM cables, but only one receiver, and it's installed in my 5160 (which is connected to my 5161).
I guess I didn't see this answered directly, but I concur, that is definitely *not* the backplane from a 5161. It's a little strange the chassis label also says "5161", normally what happens is just the lids get mixed up... but, actually, comparing that motherboard to a Googled picture of a 5160 motherboard it looks like that's not an IBM motherboard, it's a clone. So, yeah, I guess someone needed another computer more than they needed an expansion chassis.
You won't be able to use that XT motherboard as an "extender board" unless you do some serious hacking to it. (You'd need to cut a bunch of traces/remove components to turn the slot area into a passive backplane not driven by the motherboard's CPU/buffers.) You'd probably be better off either buying a new passive backplane (not too hard to find, although almost all of them are going to have AT-style 16 bit slots) or laying out a new PCB that replicates the original.
Anyone happen to know if the schematic for the backplane is out there? (Haven't looked for a technical manual for the 5161.) I'm curious about that one bit of active circuity on the board, is that a clock generator?