Mark I's pommel incorporated a so-called 'skull-crusher' cap extension, ostensibly designed to stun or kill an enemy soldier and to provide a secondary weapon in circumstances where the blade was damaged or broken. A special proprietary metal scabbard was issued with the Mark I, capable of accommodating the new knife and its oversized knuckleduster grip handle. In 1918, Captain Rupert Hughes of the U.S. Army submitted a patent application for a specialized automatic-opening trench knife of his own design, the Hughes Trench Knife. This was a curious device consisting of a folding spring-loaded knife blade attached to a handle which fastened to the back of the hand and was secured by a leather strap, leaving the palm and fingers free for grasping other objects. Pressing a button on the handle automatically extended the blade into an open and locked position, allowing the knife to be used as a stabbing weapon. The Hughes Trench Knife was evaluated as a potential military arm by a panel of U.S. Army officers from the American Expeditionary Force (AEF) in June 1918. After testing the board found the Hughes design to be of no value, and it was never adopted. Hughes went on to patent his automatic trench knife in 1919, though it appears to have never interested any civilian manufacturers. The U.S. Army adopted the M3 fighting knife as a replacement for the Mark I in 1943. The Mark I required strategic metals to produce and was too costly to place into mass production, and had been criticized as being unsuited to more modern styles of hand-to-hand knife fighting.Blending Metals to Arm Our Fighting Men, Popular Science, Vol. 142 No. 6 (June 1943), p. 104Cassidy, William L., The Complete Book Of Knife Fighting, , (1997), pp. 47-48 As the U.S. Catalog of Standard Ordnance Items of 1943 explained: "The Trench Knife M3 has been developed to fill the need in modern warfare for hand-to-hand fighting. While designated for issue to soldiers not armed with the bayonet, it was especially designed for such shock units as parachute troops and rangers." The M3 was first issued to U.S. Army soldiers in 1943, with the first knives going to elite units such as airborne troops and the U.S. Army Rangers. Despite being designed for hand-to-hand warfare, the M3 did not receive universal praise as a fighting knife upon issue to combat units. While well-balanced (some paratroopers and