Manufactured around 1964, close to the very beginning of Colt producing the ArmaLite design, the serial number of this selective fire AR-15 puts it between the first two known blocks of military test/contract production; the original recipient of the rifle is unknown, though the preceding and following blocks are known to have gone to both the U.S. Air Force and U.S. Army, the Advanced Research Projects Administration's Project AGILE for asymmetric warfare trials, Navy SEAL and Army advisor units in Vietnam, as well as American law enforcement and foreign export sales. While the launch was imperfect and not without missteps, the AR-15 set the pattern that has defined American infantry weapons through the end of the 20th century and well into the 21st, as well as the yardstick for striking a balance between power, weight and human factors in long arm design. Additionally, this example has been outfitted with a belt feed mechanism; while not explicitly marked, the conversion components match a unit developed by noted NFA manufacturer Jonathan Arthur Ciener. Post front and windage adjustable two-leaf peep rear sights, with a three-prong flash hider and M2 folding bipod on the unmarked heavy barrel, bayonet lug on the front sight base, ribbed round handguard, checkered pistol grip and a fixed buttstock with ribbed plastic buttplate. The upper receiver lacks provision for a forward assist, with a short casing deflector, and the lower receiver has the AR-15 nomenclature, caliber and serial number on the magazine well, address further back on the receiver (properly spaced around the sear pin), and a right-handed selector with "SAFE", "SEMI" and "AUTO" markings. The upper, lower and bolt carrier on this example have been modified heavily to accommodate the aforementioned Ciener-style belt conversion, with an actuator arm in a dovetail slot in the upper receiver, which is controlled by a raceway in the bolt carrier, the feed mechanism occupying the space of the magazine well, and a combination ammo bin and belt link catcher hanging below the feed mechanism. The carrier is chrome finished and unmarked, with a retaining pin slot rearranged to be clear of the raceway, and an "M/P" marked chromed bolt head.