This mainboard is labelled "OPTI-486WB REV 1.1"
The closest (by name) I found is this one: http://www.amoretro.de/2012/03/opti495-486wb- … rboard-isa.html
But the layout is completely different.

Here is a picture:

There was no CPU plugged, I guess a 486 SX/DX -33 was in place. There were no jumpers set, probably all robbed 🙁
Before plugging a cpu I checked pin C3 but there was no clock.

This is what I found out using a scope and a multimeter:

JP2 pin1: 486 A13 n.c. (DX/SX)
JP2 pin2: C493 pin 24: NPERR
JP2 pin3: 486 C14: FERR (DX) / n.c. (SX)

JP3 pin1: C392 pin 95: NMI
JP3 pin2: 486 A15: IGNNE (DX) / NMI (SX)
JP3 pin3: C493 pin 21: IGERR

JP4: C392 pin 95: NMI
JP4: 486 B15: NMI (DX) / n.c. (SX)

CPU selection DX/SX (guess)
DX: JP2 2-3; JP3 2-3; JP4 close
SX: JP2 open; JP3 1-2; JP4 open

JP5: clock selectl: 1-2=66MHz / 2-3 33MHz

JP6 pin1: ?
JP6 pin2: ?
JP6 pin3: GND
JP7: GND / ?
JP8: / ?

JP9/10/11 probably tag ram size?

JP12: GND / 8042 pin33 (display select?)

I was brave enough to power it on and it even boots from floppy, but the BIOS gives a warning: Cache error - do not enable cache.

Does anyone have more information on this board?