The Dead Air Odessa Pistol Pistons are used to adapt the Odessa-9 silencer to different handguns. Which one you choose depends on the thread pitch of your firearm. Allowing the gun to unlock without acknowledging the mass at the end of the barrel, the piston also insures functionality of your weapon.
The piston is what resides in the recoil booster, and provides the threads that are used to actually attach the suppressor to your host firearm. This piston is specific to a barrel with 1/2" by 28 threads.
Pistons are bit misunderstood, but needed for pistol based suppressors.