So, after spending way more time with this than I should have, I decided to remove the mobo from the case and have a closer look, and this is what I found:

Looks like it was done at the factory. Pin B13 was connected to pin A11. I'll let you guys figure out what the effect of this bodge is.

After desoldering the wire, the CPU is recognized as an IntelDX4-WB and CPUCHK reports write back as enabled. 😎

Good job! On non-WB processors, B13 is the CLKMUL pin so I think this was done to force a X3 multiplier to whatever CPU this board had previously installed...

Doesn't pulling B13 to ground force an X2 multiplier? Or rather, pulling B13 high (or letting it float) forces an X3 multiplier on AMD DX/2-80 CPUs, I believe. Of course, on WB-CPUs it disables WB.

So the mod could've been done to prevent AMD DX/2-80s from running at 120 MHz which would make them run hot and unstable, if at all ... maybe they forgot a jumper setting to pull B13 low.

Anyhow, this is the same CPU that originally came with this board. I got it as a complete system and the label on the case says DX4.

I could also imagine that ECS decided to disable write back because it causes problems with AMD CPUs, because the AMD implementation of WB didn't work with busmastering, causing problems when writing to floppy and with SCSI adapters, etc.

You are right, A11 is a Vss pin (and not Vcc as I assumed).