The Grenade, Rifle No. 68 / Anti-Tank was a British anti-tank rifle grenade used during the Second World War and was one of the first operational weapons to utilise the shaped charge principle. Design British research into shaped charges in the inter-war period brought about some useful progress but did not result in any weapons. Development of the grenade started in late 1939, after a demonstration arranged for a British military commission of a series of anti-tank rifle grenades and artillery warheads at the Swiss Army Proving Ground at Thun in Switzerland, which had been designed by Henry Mohaupt. Guessing that Mohaupt's weapons were using the hollow charge principle, the British abandoned negotiations and began development of a rifle grenade based on their own previous work. The No. 68 has some claim to have been the first high-explosive anti-tank (HEAT) device in use. The design of the warhead was simple and was capable of penetrating 52 mm (2 inches) of armour. The fuze of the grenade was armed by removing a pin in the tail which prevented the firing pin from flying forward. The grenade was launched from a rifle cup. The simple fins gave it some stability in the air and, provided the grenade hit the target at the proper angle (90 degrees), the charge would be effective. Detonation occurred on impact, when a striker in the tail of the grenade overcame the resistance of a creep spring and was thrown forward into a stab detonator. The grenade was fired, as was the No. 36M Mills bomb, from a specially adapted Lee-Enfield Rifle No. 1 EY, often converted from a rifle that was unfit for general use and had been marked "DP" for drill purpose. The "EY" suffix (for "EmergencY") indicated that it should be used to fire standard "ball" ammunition only in an emergency situation. The converted rifles were strengthened by adding an extra bolt to secure the breech mechanism to the stock, and by a cord binding which was fastened around the forestock. A special Ballistite high-explosive cartridge was required to propel the grenade. The grenade itself was held in place by the 2½ inch (63.5 mm) discharger cup No. 1 Mk I. With the weapon's introduction into Home Guard use, the Adaptor No. 1 and the Discharger No. 2 Mk I were introduced, which allowed grenades to be launched from the M1917 Enfield rifle with which