This is a Holosun Technologies Red Dot known as the 515CM, model HS515CM.
The HS515CM is a micro red dot optic. The operator can switch from a 2 MOA dot to a 65 MOA circle dot with a push of a button. Holosun's highly efficient light emitting diodes and solar panels allow Holosun solar optics to run without a battery in many lighting conditions. Additionally" the back-up battery lasts up to 5-years on the dot and up to 2 years on the circle dot. The U signifies a housing design that shrouds the windage and elevation turrets on two sides. This product includes clear flip-up lens covers" a quick release mount and a kill flash filter.
Features:
Parallax free
2 MOA dot and a 65 MOA ring
Powered by both battery and solar
Includes Flip-up lens covers and Kill Flash