You have a 1219 that was designed for sale outside of the US. You can tell by the switchable voltage on the power junction box, as well as that small lead weight screwed on the bottom of the chassis. That's there to reduce the "ringing" of the chassis. When United Audio imported them, they had Dual do away with that step.
Anyway, I've never seen a 1219 set up like yours with a separate ground wire - that's a US only thing (another long story - UA said American consumers would not buy a table without that wire). What was done everywhere else was to use in integral ground wire. Remove the cover on the muting switch (that box on the bottom where tonearm wiring goes). It snaps off. Look on the left hand side of the switch. Do you see a jumper wire soldered to the two left hand terminals, and then continuing on to that 3eared post screwed to the chassis through the muting switch bottom plate? If you do, that's your ground, and ground is achieved through the RCA jacks. If you hook up that separate ground wire as well, you get ground loop.
