During WWII, Cal .30 machine guns were converted to permit the use of Cal .22 ammunition (22 LR) for training purposes. One such conversion permitted the use of a conventional fabric belt. A primary component of the conversion was a Cal .22 Cartridge Holder . Photo on left shows the 3 parts of the unit. Clip, Belt Adapter, and Shell Holder. Next to them is an assembled unit with a Cal .22 cartridge in the Shell Holder. Photo on right shows two of the Cartridge Holders clipped in a belt. In use, the Shell Holder is extracted from the Belt Adapter and fed into the chamber for firing. The Belt Adapter remained in the belt, for re-use with a loaded Shell Holder.
Collectors will often find individual parts of a Cal .22 Cartridge Holder and not realize what they are. Now you know.