The cable you are looking for is the 12Gbps SAS Mini-SAS HD to Mini-SAS HD cable, which is used to connect the PERC H730 or H740 Mini Mono to the backplane in the Precision 7920 rack workstation. The part number for the cable is 0W13M, and it can be purchased from Dell or authorized resellers.
I want to install either a PERC H730 or H740 in my Precision 7920 rack workstation (I don't want to use up a PCIe slot).   The DELL documentation lists this as possible and the motherboard has the mounting posts next to riser 1A.  However, I am going to need a cable from the board to the backplane.  I have found several which all appear to be for PowerEdge servers but not managed to find the one I need for the 7920 rack.  Can anyone please tell me the part number?  
Mini Mono PERC Controller for 7920 Rack