is provided with enlarged generally elliptical openings 11 through which pass screws 13 for securing the front plate to the back plate 6 the englarged openings 11 permit adjustment of the position of the front plate 10 in the direction of arrow a a clevis like bracket in the form of a pair of outwardly extending parallel flanges 15 is provided on the outer surface of the front plate 10 for receiving an arm 16 extending outwardly from the casing 1 the arm 16 is secured between the flanges 15 by screws 18 extending through enlarged openings 20 in the arm whereby arm 16 and the casing 1 can be moved vertically a vertical bar 21 is welded to one side of the back plate 5 and supports one half of a bracket generally indicated at 23 fig1 for supporting the lower end of the casing 1 the bracket 23 includes two generally c shaped portions 24 and 25 one of which is connected to the bar 21 and the other of which is connected to the first portion 24 by screws 27 by loosening the screws 27 the vertical position of the casing 1 and its associated elements can be adjusted the portions 24 and 25 of the bracket 23 define a rectangular opening 29 having inclined front and rear surfaces 30 and 31 respectively permitting passage of the.