Puzzles in this category are often very difficult. In almost all cases this is because you can't see what's holding them together. Puzzle locks with modified internal parts or often, just the way they were made to provide an additional level of security for whatever was being locked up, provide a puzzle for the collector. The latter was common many years ago when secure locks were not easy to come by. There are still some made in less developed countries.
In the mid-1800's, match safes were common. They were used to prevent pocket fires and protect the matches from moisture. The variety of match safes was enormous. Many of them came with ingenious locking mechanisms. For many collectors, they are historical puzzles. Even knives often have ingenious blade locking mechanisms that make them interesting to many puzzle collectors.
Secret opening boxes have been around for a long time. Those made by Akio Kamei from Japan are sought out by collectors as the ultimate in secret opening box design, art and craftsmanship. Frank Chambers from Ireland makes beautiful secret opening boxes from scraps of Corian, an artificial stone used for countertops.
Almost everyone has had a take-apart puzzle in their hands at one time or another. Maybe it was designed as a puzzle, maybe it was just the battery door on your child's latest electronic toy. Believe it, or not, some of us enjoy this kind of thing.

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