Repaired our front door's 1920's Sargent Co. mortise lock on Monday. It took me 3 hours to put it back together. It actually started with a loose door knob, which became confusion of how the arm stays in, which then became curiosity of what the lock looks like. WHICH THEN became 3 hours of frustration and cursing... My advice that applies to everything in life? That piece that you convinced yourself is not upside down, is upside down.