There are numerous mentions to Z80 as well as C64, and the arcade hardware was Z80 and this project started life as a plug-in compatible board, so presumably it still has support for being driven by Z80.

If it started life as a Z80 board, probably the C64 interface is just synthesising _RD, _WR and _MREQ (and maybe _IORQ) that a Z80 design would need from the RD/_WR, clock and high address bits on 6502.

8 data bits, write signal, reset address state logic. Each byte written will prime the 24 bit address to write to in the hardware and then store bytes with auto-incrementing address. This lets the C64, or any other machine capable of generating those signals, to write large amounts of memory to the hardware.