HP 4700 CP4005 HP RM1-1785-000CN - Duplex Feed Assy for ETB - Mounts to the ETB.   Works with the Color LaserJet 4700 Printer Q7491A - Q7493AR Color Lase HP Color LaserJet 4700 Sewries and the HP Color LaserJet  CP4005 Series printers.  

 HP RM1-1785-000CN Duplex Feed Assy for ETB - Mounts to ETB

Note:  A non duplexing model with need RM1--1784 and RC1-5043 to make it a duplex.  Please look at the HP 4700, HP CP4005 Duplexer kit which includes all three parts.  

http://www.printerstop.com/paper-handling-options-8/hp-4700-cp4005-duplex-upgrade-kit-1104.html