The IBM Slide Rails are designed to be compatible with both round and square holes. The slide rails have a mechanism that uses a set of spring loaded tapered pegs that extend from the rear of the Rack Uprights through to the front flange of the Slide Rails. The taper of the pegs allow them to fit into the EIA Standard Square Hole.
The following video shows just how easy it is to install IBM Slide Rails into an APC Netshelter rack. Once installed the slide rails are as secure as they would be if they were installed into an IBM Server Rack.