The entire unit consisted of 3 parts. The shell holder, the belt adapter, and the clip. You are missing the clip.
The 22 LR cartridge was seated in the shell holder which, in turn, was inserted in the belt adapter. The loaded unit was then inserted in a special web belt and the steel clip snapped over the little tit on the adapter to hold the entire unit in the belt. (The clip actually held 2 units in place) In operation, the holder was extracted from the adapter and fed into the chamber for firing. The adapter remained in the belt.
This seems like a complicated way to use 22 LR cartridges to train MG users but I guess it worked. As far as I know the device was officially known as the Cal. .22 Cartridge Holder.