This is the CCM Valve Retaining Nut.  The valve retaining nut prevents the valve body from moving while inside the lower tube.  It is designed to work with all CCM markers, except for the SS-25, and is also found in CCM autococker and half block style lower tube kits.

   CCM products are designed and manufactured in the United States and are machined from high quality materials such as 6061 air craft grade aluminum, 300 series stainless steel, and durable plastics such as delrin and lexan.
MADE IN THE USA