Strike Industries FSA Single Side Folding Stock, Fits 1913 Interfaces, Matte Finish, Black SI-FSA-S-STOCK You may already be familiar with the Strike FSA Dual" a modular folding arm solution that can fold to the left and the right. While the new Stock Single Folder still utilizes the same arm and stock end pieces it has a strengthened hinge that's made from steel" instead of aluminum" making it extremely robust. The fitment is also perfect with little to no play when extended and folded. This folder only folds to a single side but can do so at any time without any reconfiguration or use of a button.