NES 2.0 Mapper 270 is used for the Game Prince RS-16 plug-and-play console's multicart. It uses VTxx OneBus bankswitching all similar to Mapper 256, but has an additional bankswitch register.
Outer Bank Register ($412C)
$412C is a general "UIO output data" port on the VTxx consoles. In this case, the value written to this register selects an 8 MiB base address (A23) for all PRG-ROM and CHR-ROM banks.