Adding more photos, you can see 74F245 is onboard and wired to these two buffers.

Ok, it might be 74F245, latch version of ttl.

In the end 74F245N (SN74F245N Texas Transceiver LED Driver 8-Bit-Bus DIP20) seems to work, but now I have a problem with cache. I used 20ns cmos sram for both tag and cache: Alliance AS7C256-20PC. I know it supports only 128kb of cache total. I have around 20 chips total and tried to put them randomly, maybe some of them are faulty. Once it did boot into bios and it did show 120kb. But then it locks up during the boot from HDD or FDD. Then I tried different chips combinations and it never works anymore. It beeps or it locks up after BIOS POST.

Any clues? Try to order new chips? 20ns should be safe value for 486DX motherboards.

The 5th socket "TAG" is for TAG-RAM.
The chipset might not be happy with a 256K SRAM chip in this position but demand 128K TAG (or 64K) instead.

I have additional photo from E-Bay.

Tag ram is MCM6265CJ15 = 15ns, probably 64k (this could be it!)

Cache looks like IDT chip 20ns (some have Alliance chip 20ns)

So that should be correct. I have tried tag 20ns (Alliance) and 15ns (Winbond) and no luck so far.

I have ordered 1x Winbond 15ns and 4x IDT 20ns now.

EDIT: Tag looks like 8K x 9 Bit Fast Static RAM

So I guess it won´t work with 32Kx8 for TAG...

Yes, looks like I made a progress. Just ordered MCM6265CJ15. Thank you!

Hope you get it working. I just looked up on UltimateRetro just in case there was any information on it:
https://www.ultimateretro.net/en/motherboards/1593
https://www.ultimateretro.net/motherboard/manual/33432.pdf

And... no. The MicroHouse sheet shows both TAG and U55&57 as unidentified. So if you do get it working this could be a small gap filled in.

Many / most boards have 8Kx8 for TAG with 128K cache - 8Kx9 is quite unusual. But nothing is impossible when it`s a DELL 😉

Yes, it is strange. I have read that Dell & IBM motherboards of that time could have similar compatible stuff. Here is datasheet for that TAG: https://www.alldatasheet.com/view.jsp?Searchword=MCM6265CP15 and it says 8K x 9.

I have verified that U55&57 are 74F245N and TAG is MCM6265CP15. You can see that on the last screenshot above.

I confirm MCM6265CJ15 for TAG solved my issue and now external cache is 128kb - fully recognized 20ns cache!

Windows 98 is very slow though. Perhaps need better CompactFlash adapter or this IDE controller is just ISA integrated with no DMA. Then I think only ISA SCSI can rescue this situation bottleneck.