A sheet metal support cage structure within a computer has an inturned side wall ledge portion and removably receives a spaced series of expansion cards. Extending along edge portions of the cards that face the ledge are sheet metal mounting brackets having outwardly bent end tab portions that overlie and engage the side wall ledge. The tabs are removably held in place on the ledge by an elongated hold-down bar secured at an inner end thereof to the cage structure for pivotal movement relative thereto between a closed position in which the bar overlies the tabs and captively retains them on the ledge, thereby locking the expansion cards in place within the cage structure, and an opened position in which the bar is moved away from the tabs and releases them from the ledge to permit the expansion cards to be removed from the cage structure. A single screw member is captively retained on the outer end of the bar and may be threaded into a corresponding opening in the ledge to releasably lock the bar in its closed position.