Based on traditional marine hardware. Hand carved master. Lost wax cast in Rhode Island. This kind of mechanism is also referred to as a gangent hook, kissing clip, or brummel hook. When I was a kid my father first showed me how to make them using bolt cutters and chain links. The sides pass through each other when the slots are lined up precisely perpendicular to each other. It takes a little practice at first, and soon becomes natural. Because there are no moving parts to wear out it is also well suited for good ole’ fidgeting.
The basic principle: