These are small metal jump rings with a split in them, much like the ones used in jewellery. On a model they serve as eyes and links: joining two ends of a chain, forming an attachment point for rigging, becoming an eye on a hatch, a hanger for stowed tools on a deck, or the link between a towing eye and a hook.
They come as a mixed assortment of several sizes in one pack, so there is usually a ring close to whatever hole you have drilled.
Open a ring by twisting the two ends sideways past each other with a pair of tweezers, never by pulling it apart across the gap, or it will never close without leaving a visible slot. Once closed, secure the joint with a small drop of cyanoacrylate, and degrease the assembly before painting, since nothing stays on greasy metal. Chain and rope and thread are the usual companions on ships and on vehicles alike, and the work goes much faster with proper tweezers.
