The existing TAG chips on your board UM61416K are 16kx4, the same as the board on retroweb, and in the chipset spec cheet. 16kx4 is normal for TAG RAM.

UM6164K (data) & UM61416K (TAG) are the default chips listed on the chipset spec sheet: https://www.datasheetarchive.com/?searchType=0&q=UM82C481

You could try to match the jumper settings to the board on retroweb, and/or replace the chips with the same type on your board, or on the retroweb board....

Do you know whether the jumpers are set correctly in the top-right corner of the board?

Hi MikeSG! Thanks a lot for your answer.

The cache configuration jumpers are ok. The information for the configuration is printed on the motherboard's PCB (you can see it in the first of my photos). However, I hadn't noticed that the other jumpers are positioned differently. I'll look into that.

Another question. If I wanted to expand the cache to 256k, I imagine I'd need to install 32kx8 chips.

But should I replace the TAGs? Or just put one in the empty socket? What do you think? I imagine there's no hard and fast rule...

Cheers

Bad Cache chips and Sockets is a known problem with these old motherboards.
I would not stress to much over it as long as you can get 64k to work you are okay.
With 256k working you only get about a 3% CPU boost in performance
Which is minimal.

If you really want a performance boost upgrade the CPU.

I would not worry about cache and instead upgrade the CPU and add 16mb of Memory or more.
A fast CDROM and CF card for a hard drive.

Thats where I would put my Money for Best Performance.

But if you are a perfectionist then you want to fix thoses Cache chips and sockets.
But it’s NOT going to improve your computer performance but only by a little 3% at best.

From my other thread (UM481/UM491 "Always Dirty" modification HOWTO)

Use a multimeter. The 16Kx4 socket with conductivity between pin 38 of the UM481 (ALTWR#) and pin 12 of the SRAM (WE#) is the Dirty RAM. I do not know whether it is the third 16Kx4 chip that you show, or if it is empty U12.
The 16Kx4 sockets with conductivity between pin 39 of the UM481 (TRWR#) and pin 12 of the SRAM (WE#) are the Tag RAM. If all three x4 populated SRAMs have that conductivity, then perhaps the UM481 supports wider than 8 bit tag?

The reason they used two x4 SRAMs instead of one x8 SRAM for the tag is because x4 chips came in faster speed grades than x8 at the time. 25ns is pretty slow.

Thanks everyone for the replies...