I bought these for the rear side of one of Thon's Live rack units which come, as standard, with only front rails. The rubber insets keep the nuts from sliding around when unused which is a nice touch.
For anybody wondering, the rivets have a 5mm diameter on the bit that goes into your rivet gun. Also, you'll need a 5mm metal drill bit to make the holes in the brackets big enough to get the rivet through; they are too small when they arrive but serve as a good guide. I fitted mine in about an hour Ince I'd got the bits I needed.