Those rails came with my C6100 and they suck. There's a... slide stop? at the back end that keeps the face from being flush with the rack and prevents removing Node #2. And I'm not sure those rails were intended for square-hole racks. Mine came with some screws for the top two holes but the heads aren't quite large enough to prevent them from going through the hole. Need to add washers, find screws with larger heads, or risk that the rails might fall out any time you mess with the server.
I think I have my C6100 on some MD1000 rails that I liberated from a former job.
This $25 kit looks pretty universal and snaps in to a square hole rack.