I purchased this clamp to use with my BlackRapid RS-4 Classic Camera Strap so I could very quickly connect it to my Really Right Stuff BGE11-L: L-Plate for BG-E11 Grip (5DMk3).
Manufacturers Link: http://reallyrightstuff.com/ProductDesc.aspx?code=B2-FAB-F&type=0&eq=&desc=B2-FAB-F-38mm-clamp-with-flat-back&key=it
Here's a few pics of it:
