The 747-400 upper deck door security relies on large pins sitting in L shaped-slots (plus the electronic system locking the door handle in flight). The locking system can be inoperative for flight as long as the door is guarded by the flight attendant when cabin/external differential pressures are low. With larger differential pressures, the door latching mechanism is loaded up with greater mechanical forces (making it impossible to open the door).
Thanks NSEU. That makes sense, learned something new. That is a clever design if it can't be plug type, using the air pressure to hold the locking pins, nice "passive" system to achieve the desired result.