21 and 22 in place referring particularly to fig4 7 the application module 2 is securely retained in its operating position by a fastening mechanism which includes a lever 59 the lever 59 is fastened to the bottom 26 of the application module housing 25 a bolt 60 extends upward through an annular shaped guide 61 and into threaded engagement with a metal pivot bushing 62 the bushing 62 extends through an opening in the lever 59 and it has a cap 63 which holds the lever 59 in place and allows it to pivot about a vertical axis one end of the lever 59 extends forward through a slot 64 in the housing cavity 32 where it can be manually operated by the user the other end of the lever 59 extends into the main unit 1 where it engages a cam plate 65 that is fastened to the bottom of the bezel 3 through a pair of integrally molded posts 66 the operating end of the lever 59 is bifurcated to provide a pair of spaced fingers 67 which support between them a roller 68 as shown best in fig6 when the application module 2 is inserted into the main unit 1 the roller 68 on the operating end of the lever 59 enters the mouth portion 69 of a slot formed in the cam plate 65 at this point the electrical connectors 14 17 are in partial engagement and the guideposts 21 and 22 have properly aligned the application module 2 for complete engagement the lever 59 is then pivoted in the clockwise direction and the roller 68 rides along a sloped cam portion 70 of the slot in the cam plate 65 a force directed along an axis parallel to the guideposts 21 and 22 is.