NSN 1240-01-316-0055 Since its introduction to the marketplace in 1989, this foundation has maintained its reputation as being the best mounting systems for the M21/14 Rifle. The A.R.M.S.® #18™ offers interchangeability of Day & Night Vision, with an ultra low profile and the A.R.M.S.®/SWAN ® aka Mil-Std. 1913 Rail. Made of 8620 case hardened steel. Precision fit to the base integral with the receiver. Two attaching points plus a third contact point for ultimate stability.