and 406 and the positioning hole 404 of the plate 41 are provided respectively with flanges 413 414 and 412 there are also rear engaging notches 407 and 408 in the plates 41 and 42 the space confined by the plates 41 and 42 accommodates three elements i e a transmission plate 5 shown in fig7 and 8 a front cam shown in fig9 and 10 and a rear cam 7 shown in fig1 and 12 the front and rear cams 6 and 7 are mounted on the side plate 41 and can be selectively connected to the spindle of a cylinder lock so as to create a desired backset the cam 6 or 7 transmits the movement of the spindle to the transmission plate 5 which operates the latch bolt 2 and provides two backset measurements which are respectively 60 mm and 70 mm the transmission plate 5 is provided adjacent to the side plate 42 and is a generally oblong plate which has at its front end an engaging member 50 extending into the case 1 and engaging with the bolt 2 an axial oblong opening 51 is provided in the rear portion 52 of the plate 5 to divide the rear portion 52 into two substantially parallel strips 521 having inner sides defining opening 51 the axial opening 51 is in alignment with the cam guide slots and the positioning holes at one side of each.