alignment pins 30 and 32 it is seen that mounting slots 24 25 26 and 27 may be symmetrically arranged on the edges of the equipment box 10 so that regardless of which side of box 10 is downward cam 36 will fit in one of the slots similarly alignment holes 18 19 20 and 21 as well as alignment pins 30 and 32 may be cooperatively symmetrically arranged about corresponding edges of the equipment box 10 and tray 16 respectively so that any selected side of the equipment box 10 may be mounted directly on tray 16 if for example the distance between holes 18 and 21 is the same as between holes 20 and 21 the box 10 could be mounted on tray 16 ninety degrees from the position shown with cam 36 binding in slot 25 to hold the box 10 down against tray 16 furthermore if all of the holes 18 21 were equally spaced any one of the four sides of the equipment box could be downward against tray 16 and on opposite ends thereof the corresponding mounting slots 24 27 would cooperate with cam 36 accordingly the invention permits any one of the sides of equipment box 10 to be mounted directly on tray 16 apertures formed in.