More than one year later, while cleaning around my desk and having nothing better to do, I thought I'd play with this board a bit. It didn't start as it was, but I found another photo online, set the jumpers like in the photo, and it POSTed with a 486DX2 CPU.
BIOS ID string is 41-S203-428075-00101111-101094-UMC880-H. Searching the net for this exact string yields no results, however searching for parts if it led me to this page:
http://motherboards.mbarron.net/models/486pci/pt432b.htm
Pine PT-432b is almost identical to my board, except for a couple of jumpers, diodes and capacitors in the COM port area. The manual doesn't clarify everything for me, but it's a start. For now I seem to have trouble getting the CPU (SX807) recognized properly - I'm not sure whether to jumper it as a DX2 / P24D / P24C / P24T and so far I've got 66MHz with normal CPU score in Speedsys (but missing L2 cache), 66MHz with 386DX-like score, 2MHz, 198MHz and 132MHz (it would POST and display these, but wouldn't move on to loading the OS).
It's a work in progress, I guess... at least now I have a good reason to hang on to this board 😀.