Systema PTW Switch-Device 4 (M4/CQB-R model). Miniaturized ECU replaces the stock 2012,2008, larger factory original ECU. More than 50% reduction in length! Storing this smaller ECU unit in the factory stock tube means that there is room remaining to store a LiPo battery in the remainder of the stock tube. This means that you will not have to store the regular Systema battery in the factory CRANE stock, and so that you can change to using ANY other stock on the market.
Also, please check out the "RWC Milspec Size Stock Tube for Systema PTW", a custom milspec stock tube for using any other buttstocks.