Dating antique furniture locks
A half mortise lock is exactly as it sounds—half exposed.The key must not only be the right size to move the bolt forward and back, it must be the right size to compress the spring and release the bolt so it can move.Most bolts have two notches, one in the locked position and one in the unlocked position.The levers must be aligned in a perfect line to allow the bolt to pass but since their thickness is random and hidden, figuring out a cut pattern is very difficult.
This lock requires notches to be made on the bottom of the key blade rather than on the face of the blade and is much more difficult to fabricate. Apparently, the 19th-century English had more of a need for security than we did.
This is one case where if you don’t have the key, don’t worry about it.