I just had a flash-back from when I used to work in a shop ('bout twenty years ago). People would walk up to the counter with a lunch bag and dump out an old mortise lock (in pieces).
This was in the days before Rubik's Cube.
For those of you who don't know where I'm going with this... Putting a mortise lock back together when you never got to look at it before it was taken apart is one of the pest puzzles you'll ever do. (And very profitable)
Bobby