The Mk14 Extended stock is supposed to be in Koani along with its folded variant but instead the POSP scope is in its place. As far as I can tell the Extended stock is completely absent from the game and there is no way to get it, it is not even listed in the store.
Edit: Nevermind, turns out when you pick up the folded variant in Koani it also unlock the extended variant at the same time. Still a bug though.