If the game has its original locks and keys, and the key is missing, you have no choice but to drill out the coin door lock.

The headbox key will most likely be in the front door panel, near the leg bolt plate or tilt mechanism. Sometimes the headbox key is floating around inside the lower cabinet.

If a machine has been stored on it's back, you can sometimes find the front door key in the cabinet. Just put your hand in the space where the headbox cables route through to the head.

Some operators changed the locks and replaced them with a universal lock. So one key will open all their machines.

There was no factory "master key" for any manufacturer.

Locks are cheap, so it is always better just to drill them out.