Case-mount rollers are used when you don't want any hardware mounted to the drawer. These flanged rollers screw or staple directly on top of the case frame. They come in right/left pairs, are mounted in a steel frame and provide a height offset of 0.225".
Though typically used in low-end production furniture, these rollers can be an effective, quick and affordable solution. Sold in pairs. Require #6 screws (not included).