This is a replacement part for the Echo AR-II Trigger. You do not need to purchase this item unless the part that came with your Echo Trigger has been lost or damaged. This relates to part M on the parts diagram that came with your trigger. If you no longer have the original parts diagram that was included in your Echo AR-II booklet, you can find a pdf copy of the booklet on our website. Please navigate to the home page and scroll all the way to the bottom of your screen. There is a section in the footer titled manuals with pdf copies for each Echo Trigger.