The later Dual TT's as opposed to the older models (1209 included) all had separate grounds- for a reason.
If we look at your 1209 we can see a ground bridge on the small contact plate in the back of the tonearm head, from the cart right channel ground to tonearm metal. Next we have a ground bridge from chassis to left and right shields contained in the two tonearm wires.
Let's look at these ground loops a bit closer.
All these tie points create ground loops.
The single joined ground binding point in the muting switch, going into the audio cable to amp, allowed Dual to omit putting a separate ground wire (cost?)for the Canadian market. This has one good effect in that the chassis ground is carried on both RCA connectors, so there's less risk of loosing a ground from either cart channel or the chassis, should one of the RCA ground wires fail.
This is fine and dandy if the cart has a fairly strong output and if the amp's preamp is not too picky. When we move up a bit to a more sensitive/lower output cart and/or a sensitive preamp circuit in the amp, these multiple ground points create grounding circles/circuits. In these grounding circuits we will have minute differences of potential /voltage. When the cart puts out more, then this nuisance background noise is covered. The less the cart puts out, it starts to get annoying.
The ideal way to alleviate this, is to remove all these ground junctions. Now the cart right ground, cart left ground and chassis (via a separate ground cable) go to the amp (preamp) without creating an intermediate ground loop with its potential source of trouble. Since there is no circuit/circle in the ground path, the voltages in these ground circuits can't run in circles....
Here is a good view of how Dual wired the units for separate ground wiring:
The lowest set of muting switches used in the 1200 series including your 1209 shows the left version as we received the unit here in Canada albeit with RCA end connectors but no separate ground wire. To the (bottom) right is the the same muting switch with RCA plugs. You will note that the red shorting wire from the all left switches are missing and a separate chassis ground wire is added. The factory simply added an 18 gauge wire with u spade connectors on both sides and slipped one of them under the audio cord clamp. I always soldered that end of the wire right into the muting switch ground tab to make sure it does not loosen.
This type of ground wire setup is basically what everyone else with better equipment did/does. The one thing that must however be ensured, is that each wire junction (headshell to head plate; cart to headshell) is clean and conductive.
Wiring this way has never given me any problems,but certainly eliminated some!
BTW,the short on the connector plate has not proved to be a nuisance since the distance to the cart is so small. It can be left intact.