This it the AR15 Grooved Magazine Catch Assembly from 2A Armament.
The 2A Armament Magazine Catch Assembly offers innovative styling and functionality. The grooved magazine button provides a good tactical response and is machined from aluminum with black hard anodize finish for durability, the center pin is stainless steel.
Complete assembly weight .27 oz.
Compatible with AR15 and AR10 platforms
Ships ready to install with a spring, center pin, magazine catch, and magazine button.