So here's something interesting. Looking at socket 7 boards on ebay I seem to have come across one that is aesthetically (PCB color, trace aesthetic (grid of tiny squares), silkscreen style, and more importantly the serial number/part number markings sticker color/typefaces) very similar to the one in my machine.

Searching for the ebay board part numbers, it looks like it's an Intel Advanced EV Endeavor reference board.

Unfortunately a search for my part numbers PBA 642569-205 / CA 640782-203 comes up blank but it must also be an intel board off the same line.

Could there be different riser designs that may work with this board?

edit: digging further, my board is very similar to these
Advanced/MA - Monaco
https://stason.org/TULARC/pc/motherboards/I/I … -MA-MONACO.html

ADVANCED/MN (MORRISSON)
https://stason.org/TULARC/pc/motherboards/I/I … -MORRISSON.html

Intel Pocono by a company called Advent
https://www.elhvb.com/mboards/advent/Intel%20 … Motherboard.htm
this one is very close in terms of layout. CMOS battery, cache slots, even chipset locations look right

Here's another weird thing. Does PBA mean anything in the intel world? A search on ebay for Intel PBA yields a lot of similar boards, like this NCR one. VERY similar layout. Based on S3 graphics this is probably based oin the VX chipset
NCR PBA 648694 Intel Processor Board with Riser Slot and AT + 6-Pin Power

So after a lot more research:

1- Intel did indeed manufacture a lot of boards for OEMs. Custom versions of boards for AST, Packard Bell, other big companies
2- My board is closely related to the PB640 and 660 boards. Slightly different component choice, but layout is dead on.
1.00.xx.CP0R: Packard Bell OEM version of Intel Thousand Oaks (PB640)
1.00.xx.CP1R: Packard BellOEM version of Intel Fair Oaks (PB660)

3- A lot of these LPX boards have similar looking risers.

So the question remains. What are the chances LPX risers would be interchangeable?

For example, this guy would be right up my alley.

I think "PBA" is just a prefix to an intel part code. But it seems to be consistent with anything made by Intel.

I only have 1 LPX board that uses the type of riser you're showing here, but it's an AST made board. There's another Intel made LPX board that I'll be getting soon which is without the riser card, so I have the same question about interchangeability.
If you have a look at the manual for that pentium board in section 1.10.11, it lists the full pinout for the riser card as J6E2: https://www.fermimn.edu.it/inform/materiali/e … bd/28181002.pdf

You can find some riser cards on ebay if you look for "eisa riser", since the connector is an EISA slot. Electrically, the way Intel made these is by matching up the ISA pins of the EISA slot with the ISA pins, with the PCI pins being deeper down and seemingly matching up in-order as well.

I've just confirmed that the ISA pins match up by plugging a basic ISA riser into my AST LPX board and the power pins all match up with where they should be for an ISA card to use.

What are the chances LPX risers would be interchangeable?

For the ISA pins, it's unlikely someone would want to re-design the wheel and they'd just map the EISA-ISA pins to the regular ISA pinout.
For the PCI part, well that could be different, as the EISA 32-bit bus isn't the same as PCI, so they would have to decide where pins go. But on Intel made boards, I think they'd all be the same as that pinout in the manual I linked.

Potentially, I can pull the riser out of my AST 486 and see if it matches the intel pinout for the PCI pins.

AST-AUTISMO wrote on 2020-09-11, 06:56:

[...]

So the question remains. What are the chances LPX risers would be interchangeable?

Very high - with a few catches.

Basically there are only a few standards for pinout, and the kind you have there is plain old PISA. The only possible confusion is with real EISA, but as you know your board is PISA, not EISA, and you're not looking for EISA slots, that's not an issue.

But...

Getting a riser compatible with the PISA slot is the easy bit. More troublesome is the thing that's not well standardized in LPX: the distance to the backplate. Different systems have different distances, which mean that the slots on the riser can be further forward or back compared to your existing riser/case.