Having poked about the schematic a bit, and being a long time circuit designer myself, there are many (what I would term) stupid mistakes. It seems as though the non-existant U20 (GT20SL16S1Y) could be used as either additional memory, or as a replacement for the current memory. As I said in my original post, I can’t find any reference to a part number that is even vaguely similar (which is a serial memory device).
It would not make sense that U20 would replace U13, as most SPI serial xRAM chips are 8 pins, and not 6. In fact 6 pin xRAM chips are really hard to find.
As you point out, it is wired to the same SPI bus but enabled with ZKCS. If it were to be a replacement, it should be wired to EECS so that no firmware change would be needed to access it.
I already thinking a hardware mod, using ZKCS but with an 8 pin xRAM chip soldered on top of U13… it would need a firmware change to access it though…
Oh! What is the other thread you refer to. I have not been able to find it…