Emulation-wise, there is one register at $4120 (at least that's the address that the code writes to):
Code: Select all
4120 ------pm Mode Register
m - mode
"0" - mapper 70 mode
"1" - mapper 1 mode
p - prg 64K block in 70 mapper mode
In order to achieve that, along with a number of 74LS chips, a 24-pin chip that I think is the TXC 05-00002-010 IC (marked "52S") seems to play a role in this, connected in an entirely different manner compared to Mappers 36, 132 and 173. In particular, pin 5, whose function is unknown so far, seems to be connected in a meaningful manner (to PRG pin 24). On the other hand, since pin 12 seems to be connected to GND, maybe it's just the kind of PAL that the TXC IC on the Mapper 173 board pretended to be but wasn't. Unfortunately, the whole cascade of logic gates is a bit too difficult for me to follow.
Here are three PCB images, originally taken by CaH4e3: front, front (some chips removed), back.