Apple LaserWriter 12/640 PS - Manual (Page 222)

7 Align the arrow on the left side of the toner cartridge with the arrow on the left side of the
printer, then slide the cartridge downward into the printer. Make sure the toner cartridge
is inserted all the way and properly seated inside the printer.
Make sure the toner cartridge is
inserted all the way and properly
seated inside the printer.
223 Maintenance