It is a Shield Reflex Mini Sight (RMS) made in England. It is in a specifically milled slide cut. The sight is not as wide or tall as most other red dot sights, so it fits compact guns like the S&W Shield real well. It has a sighting groove in the back, so I can use standard height Heinie Straight Eight nite sites. It's a 3.5 MOA dot and works just fine.

Shield Sights RMS in Full Production | RECOIL