From my website http://m14.freeservers.com/about.html
Experimental Items for the M14 Rifle - In 1961, the experimental breech loading X-1 40 mm grenade launcher was built for the M14. It attached to the gas cylinder and bayonet lug. Aiming was performed using the standard M15 grenade launcher sight. Two hundred aluminum M14 magazines were produced by Mathewson Tool Company and tested by Springfield Armory in 1962. That same year Harrington & Richardson produced experimental promethium flip-up rear and front night sights. In 1962, Springfield Armory developed five versions of a folding stock designated as the M14E1. The designs included both underfolding and sidefolding stocks for the M14 rifle. Springfield Armory experimented with white, black, brown, green and tan colored stocks in the first half of the 1960s during development of the synthetic M14 stock. Some disposable plastic pre-filled thirty round M14 magazines were reportedly made and tested by Rock Island Arsenal in the early 1970s.
My source regarding the thirty round M14 magazines made by Rock Island Arsenal is Mr. William J. Ricca. Mr. Ricca is a U.S. military small arms expert and surplus dealer.