Converts
"Converts" commonly refers to modified cartridges, with their ROMs replaced by those of MVS games (which are usually cheaper).
This can be done for one's own use, for selling clearly as "converts" (doubtful legality), or for plain fraud.
The usual method is to replace the label on the plastic shell, remove the memory chips from the PCB, eventually modify the jumper configuration, and solder back either MVS cartridge ROMs, or freshly programmed EPROMs containing the new game's data.