Abstract: The hold down apparatus securely engages a unit onto a tray. The tray includes an opening having a front block on the underside of the tray at the opening. The front block also includes a pin. The apparatus comprises a housing having an internal space and is placed in the unit such that the housing space is opened to an opening in the bottom of the unit. A control shaft extends from outside the apparatus and into the housing space to the backplate of the housing. The portion of the control shaft in the housing space is threaded. The external portion of the control shaft is accessible from the front of the unit. An L-shaped pawl has a foot and a rise, the foot of the pawl having a notch in the front edge of the foot.