Converts
"Converts" commonly refers to modified home system cartridges, with their ROMs replaced by those of MVS games (which are usually cheaper).
This can be done for 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 chips containing the new game's data.