I would recommend to use those gray flatbend cables to... Its much easier to disconnect them when you want to remove the extra ISA board.

Do you maybe have a picture of the other side of that ISA bus connector bus card?

I'll go the pins way to connect both isa busboards, that way removing the addon slots will be easy and clean as I'll only solder 2 rows of pins on the original board.
I only have this pic of the back of the board, nothing there except the power connectors:

Maybe some kind of backplane connector is directly solderable to the motherboard?
I believe the pitch is 2.54mm and distance between rows is 5.08mm.
Then you can just make the extra PCB with same backplane connector, and standard backplane cable between them?

^ standard pins do work , the plan is to solder 2 rows at the unpopulated ISA slot at the top of the original busboard. Then use female pin headers to connect to them.

I think an ISA riser card from a desktop machine is what you're looking for:

How do you plan to attach a ribbon cable to the headers though? They're spaced pretty far apart, I don't think there are any standard connectors for this.

Oh these are some nice busboards, where did you get them from? If I had one of these then things would be way better...
I'll post some pics later of the pins etc , I'll use single row ones

Found at the recycling centre... They're from a Hyundai 8086 machine, don't know the exact model number offhand.

^ it would be really nice to have something similar here but I can't find anything...

for now I soldered pins in the empty slot of the busboard:

and here is the general plan , requires lots of wiring unfortunately 🙁 , I'll add a 3rd slot because doing that in the future again is a big no-no

When you're done you'll feel an immense sensation of satisfaction with your hard work.

I have a spare 16 bit riser card that i could send your way but i'm in England and the damn thing is stashed at my folks place in Portugal.

🤣 yeah, after soldering 62 wires X many times and it works it will really be satisfying 😀
Thanks for the offer Carlos, I continue to search locally just in case I find a riser... what I don't like is that the final hand-made board will look like spaghetti...

so yeah.... well... I just cut a piece of a dead 486mobo I keep around to steal parts and made a ghetto-style busboard...

removed some components like 3 resistor networks and 2 small smd resistors , kept the 4 decoupling caps (they connect to all the slots at +5/-5/+12/-12 lines) and verified I have 1:1 connectivity between all slots. Doesn't look that bad, I prefer it to hand-made one 🤣
Will make some tests once I get some straight pins to solder to it.

That actually looks really good!

Extremely curious how this turns out.

Nice, man! That's a really cool idea. Certainly beats manual wiring 😀

However you may want to leave the termination resistors in place, and maybe cut all the traces that lead nowhere, to ensure signal integrity.

^ termination resistors were in the middle of the slots , the empty isa slot is where I will solder straight pins and connect this to the original busboard. Do I need the termination resistors there?
Good thinking on the unused traces, time to cut them 😀

I think resistors should be at the end of the bus, that would make sense to me. Might be better to leave them out for now, especially if the mainboard has resistors already. If you run into stability issues, just put them back in.
Maybe solder in some SIP sockets so you can easily replace the resistors again. 😉

Yeah judging from the original busboard there is no need for termination resistors... maybe they were needed in the 486 board, who knows... I'll test and see how things go

The resistor neworks were 470ohm - 10K ohm - 10K ohm , maybe they used them to power the ISA slots so there are no power/signal issues , just a guess

finished cabling today:

Unfortunately it does not work. I resoldered the resistor networks (they are used for pullups) and also tried without the decoupling caps but it didn't help.
There must be something else.
Using the extension cables I made with a quickly soldered isa slot works fine, so I think the cables are not the problem.
So back to square 1 for me... :\

1 - did you check continuity with a multimeter? As in checking between the pins you soldered and the last ISA slot on the board?
2 - are the signals reversed? As in the signal from the first contact being routed to the last one? If so, reverse it back.
3 - are the slots at least receiving power?

^ yes to all of that 😁
- each and every pin of the extra isa slots are checked against a slot on the original board , they are all connected OK.
- quadruple-checked signals to be 1:1
- slots are receiving good power , +5v / -5v / +12v / -12v

I think there is at least one board plane inside the slots from the 486 mobo, maybe it created interference to the signals since the rest of the board is missing 🤣
For now I think that an ISA riser like jwt27's or hand soldering is the only way...