same as the clamping mechanism 44 in arm 22 preferably individual clamping mechanisms 96 will be provided for each rod 34 and 36 and the plate 46 for each will be mounted and sized to contact only its respective rod the jaw member 94 is provided with two ball detents 53 spaced as indicated in position to contact the peripheries of the rods 34 and 36 and align them the use of two spaced ball detents 53 in the jaw member 94 is to accommodate the smaller diameter of the rods themselves as compared with the couplings 66 which encircle the rods comparing fig4 and 6 it will be apparent the spacing between the jaw members 40 and 42 or width of slot 38 is larger than the spacing between the jaw members 92 and 94 or width of slot 90 and the depth of the slot 90 is less than that of the slot 38 because of the differences in outside diameters of the rods and the couplings in operation two rods 34 and 36 are mounted in side by side position with their longitudinal axis substantially parallel and with the couplings 66 at one end of the rods received and held in the slot 38 between the jaw members 40 and 42 of arm 22 by detents 53 the rods are received and held in the slot 90 between the jaw members 92 and 94 of.