As shown in FIGS. 3 and 4, the electrical contact matrix 39 mounted beneath or integral with the plate 31 consists of an insulated board 50 of any convenient insulating material, such as LUCITE, a plurality of parallel column contact strips 51, recessed into the upper face 52 of the insulated board 50, and a plurality of parallel row contact strips 53, recessed into the lower face 54 of the board.