A word from the designer:
Two keys appear to be permanently linked and, no matter how you look at it, there is seemingly no solution to unlocking them. However, as a highly popular puzzle since its creation during the 19th century puzzle craze, the keys are in fact separable.
The hint is "Enlarge".
Puzzle Master makes a large assortment of disentanglement puzzles, brain teasers, jigsaws, puzzle rings, trick locks, boomerangs, chess sets, etc. This Canadian company specializes in taking traditional puzzle designs and adding their own unique touch.