The Trijicon RMR is designed with a "gasket" on the underside that can be sealed with this plate.  Should you find yourself wanting to swim with your pistol, this part is a must.  It will sit between the optic and your mounting surface to form a waterproofing seal.  Made in house by Danger Close Armament and finished in black.  

Features:

• Allows the optic to be submerged up to 66ft 
• Compatible with all battery powered Trijicon RMR models
• Made in the USA
• Professional Installation Recommended
• Weight: 0.1 oz