The Nikon K-ring set is a stack of tubes and adapters that gives you the opportunity for all sorts of F-mount shenanigans. The set includes the K1, which is a short F-to-F extension tube; the K2, with a lens-side F mount and an inner 52mm thread; the K3, with a 52mm outer thread on one end and a camera-side F mount on the other; and two 52mm threaded extension tubes. In its simplest form, it's an adjustable extension tube, but there are enough odd configurations to keep a set on hand in case you need to do something weird. In good working order, slightly worn cosmetic condition, with leather case.