0 015 inches thickness where they join the side walls 76 the hinges provide a predetermined amount of resilience which is sufficient to support the catches 56 and yet permits the catches to bend the arms downwardly in response to an applied magnetic unlocking field the spring arms 78 extend from locations on the inner surface of the side walls 76 midway along their length and about 0 040 inches from their upper edge the spring arms slant upwardly as they extend from the side walls so that their outer ends are about 0 020 inches below the upper edges of the side walls 76 the catches 56 are each formed from a ferromagnetic material of sufficient hardness to hold the shank 66 of the fastener pin 62 securely without appreciable wear preferably the catches 56 are formed of sae 1010 cold rolled steel which is heat treated to form a carbon nitride casing and then nickel plated in the illustrated embodiment the catches 56 are 0 036 inches thick the wing portions 58 are flat generally rectangular sections about 0 344 inches long by 0 136 inches wide with a rear edge 82 and a front edge 84 a gripper projection 86 extends out from the center of the front edge 84 to a distance of about 0 188 inches from the rear edge 82 the front of the gripper projection 86 contains an elliptical recess 88 which forms a gripper surface to grip the shank 66 of the fastener pin 62 in the illustrated embodiment the elliptical recess has a major axis parallel to the front edge 84 of 0 048 inches and.