The Device was designed to fill a need for a strong, stable fixture for securing the Upper Receiver of AR platforms (as well as similar platforms that have an upper/lower combination) for the purposes of all manner of build, rebuild, accessorizing and maintenance. The Device was to hold the upper receiver in a fast, intuitive, secure fashion that would allow for the functioning of parts attached to the Upper Receiver, and be utilized as a conduit for rear sight spring alignment on A2 variations. Also, this one large pin concept would do away with the traditional approach of utilizing two small pins, one in front and one behind, common in other style fixtures. This would allow for one of The Devices intended, and highly advantageous, purposes; the clearance cut that allows most Rail Systems complete access for installation. As rails will generally not protrude further rearward than the leading edge of the front receiver lug, a clearance cut in the main body of The Device allows sufficient room for even the most rearward protruding Rail Systems. The rear of The Device has a relief cut for the removal of the forward assist roll pin. This, again, is able to be achieved due to the one large pin concept. The Upper Receiver is held through the receiver flats, interiorly, as well as by the lugs. This allows for a very strong hold on the Upper Receiver. The Device does not interact with the Barrel or Barrel extension. This keeps the likelihood of Barrel damage to a minimum and allows for alternative caliber support such as 9mm. The Device will allow access to the entire exterior of the Upper Receiver, with or without a barrel, with or without optics or other accessories. This overall access allows an Armorer/Gunsmith flexible work conditions and capabilities.  The underside of The Device has a dimensionally accurate M4 type  1913 rail. This allows great flexibility in optics and accessories set-up, repair and maintenance. If the object attaches to a 1913 rail you can attach it to The Device. The Device is provided with a provision for a lanyard to keep the upper and lower portions of The Device from being lost or damaged due to dropping. An interior ball detent positively holds the upper portion of The Device onto the lower portion. This allows for vertical employment of The Device without fear that the upper portion sliding out of the receiver thus losing control of the Upper Receiver. The Device is proudly made in the United States of domestic 7075 T6 aluminum that is Type III hard coat anodized. The Device is designed to give a service life far exceeding other designs. This will likely give most end users a lifetimes worth of work. The Device was awarded a U.S. Patent in 2012. The Mk1Mod0 Device is for use with the AR15/M16/M4/416 variations that are within MilItary Specification tolerances. Billet Upper Receivers with interior and lug dimensions within Military Specifications will also function as the exterior surfaces (sans the very bottom) are not touched (unlike with clamshell style designs). Other variations, such as an M110 variant, are forthcoming. The design of The Device allows for adoption flexibility for many styles and variations of platforms.