Magpul makes high quality injection molded parts and accessories, primarily for the AR15 rifle platform, right here in the USA. Polymer backup sights are popular for several reasons, including low weight, quiet operation, and durability. High quality polymer can take a certain amount of abuse without breaking, and still return to its original form, unlike metal, which may permanently deform, or even break.
The MBUS (Magpul Back-Up Sight) is a low-cost, color injection molded, folding back-up sight.
The dual aperture MBUS Rear Sight is adjustable for windage and fits most 1913 Picatinny rail equipped weapons, but is specifically tailored to the AR15/M16 platform.