This is the Holosun HE403B 2 MOA green dot sight.

The HE403B-GR is a 20mm micro-sight designed for rifles and carbines. Made from durable 6061 aluminum and includes a 1/3 co-witness mount (1.63″) and a low mount (.75″) for low-comb height rifle applications. Plus, digital buttons for changing settings.

HE403B is packed with Holosun innovations:

• Shake Awake
• Super LED with up to 50k hour battery life
• Multilayer Reflective Glass
• 12 Brightness settings: 10 daylight options and 2 night-vision compatible options
• Parallax-free & unlimited eye relief

Comes with the following:

• Lower 1/3 cowitness mount
• T10 L key
• User Manual
• Lens Cloth
• Low Profile Mount
• Rubber Cover
• CR2032 battery