[CA11] RD218 and the C120

Can anyone help bring some clarity for me? I have a C120 installed in my SC35 and want to add a compatible radar. The RD218 is stated to be compatible on the Raymarine site. I have found some new ones but they are described as being either E52079 or E52065 - does anyone know the difference - will either of them work with the C120? I did find a note somewhere saying that the RD218 serial number needed to be 530158 or higher to be compatible (naturally one that I have found is 510056!).

Also...any advice on running the cable from the arch? I can see where the ceiling panel is removed and could probably remove the STBD speaker to pull the cable down. From then on....any suggestions? Thanks