NES 2.0 Mapper 321
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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.
Outer Bank Register ($6000-$7FFF, write)
A~[011. .... ..PP MB..], A001.7=1, A001.6=0 || |+---- PRG/CHR A17 || +----- 0=PRG A13..A16 from MMC3 || 1=PRG A15..A16 from PP, || PRG A13..A14=CPU A13..A14 || (NROM-256-like PRG banking) ++------- PRG A15..A16 is M=1