will lock the monitor in place in the interior 34 b of the housing 34 an alternate arrangement of securing the monitor in the housing is shown in fig6 a and 6b in this embodiment slide switch 70 is movable in opening 72 in the direction of arrow 74 extending downwardly from the switch 70 is member 76 which cooperates with a slide member 78 biased by spring 80 to project outwardly from interior sidewall 34 c of the housing as shown in plan view of fig6 b the member 76 engages the sidewalls of a groove 82 of the slide member 78 upon sliding of the switch 70 in opening 72 member 76 will engage the sidewalls of the recess 82 and force the slide member 78 rearwardly against the bias of the spring and into the interior of the sidewall of the housing the monitor 32 may then be lowered into the interior 34 b of the housing upon movement of the switch 70 in an opposite direction the member 76 will be withdrawn from the groove 82 and by the bias force of spring 80 projecting member 78 a will extend into a corresponding recess not shown in the sidewall 32 b of the monitor when the monitor is fit into the interior opening 34.