This stock is one of the default accessories for the real-life H&K MP7A2. The name of this attachment is inappropriately used, as MP7 models all uses collapsible buttstocks akin to the Heckler & Koch MP5, not folding stocks (i.e., not unfolded, but extended).
The release latch that allows the stock to collapse and retract appears to be missing. This modeling oversight accidentally justified this being an expensive upgrade, since the whole ordeal might've well required a black market gunsmith to pull the gun apart and permanently fix the stock.
The Unfolded Stock is not included in the SpecOps' modification showcase animation on the DLC's announcement site.