NES 2.0 Mapper 321 denotes the 820310 MMC3-based multicart circuit board, UNIF MAPR BMC-820310-C. It is basically NES 2.0 Mapper 287 looking at differently-ordered address bits.
A~[011. .... ..PP MB..], A001.7=1, A001.6=0
|| |+---- PRG/CHR A17
|| +----- 0=PRG A16..A13 from MMC3
|| 1=PRG A16..A15 from PP,
|| PRG A14..A13=CPU A14..A13
|| (NROM-256-like PRG banking)
++------- PRG A16..A15 is M=1
Categories: MMC3-like mappers, Multicart mappers