INES Mapper 250
INES Mapper 250 is a mapper by Nitra, used for pirate games such as Time Diver Avenger and the Queen Bee V.
FCEUX implements mapper 250 as an MMC3 variant where instead of using the LSB of the address to determine which register to write to, it uses the $0400s bit (i.e. the registers are at $8000, $8400, $A000, $A400, $C000, $C400, $E000, $E400 and their associated mirrors).
Per Nintendulator, the provided value to write is discarded. Instead, the value written to each register is the lower byte of the address (i.e. address AND $00FF).