As mentioned above, the external keyed driver's-side door-lock was a factory option (not standard) that was only available for doors with non-moving outside door handles (52-55).
The inside door handle was used to lock/unlock from the inside the driver's-side door on 47-55 trucks.
You either push the handle forward or pull it backward, to either lock the door or open the door from the inside of the cab. The direction for moving the handle for locking the door changed during the 1948 model year: pull the handle to open; push the handle to lock.
