Along with many others over the years I have been frustrated with residual hum from my Decca cartridges; previously a Blue and latterly a Supergold.
After endless cable swaps, equipment moves and ground wire configurations I managed to get it down to a tolerable level but it never went away and I still got hum when I touched the headshell.

I just completed a new project rebuilding a TD 150 and installing a Mission 774 arm specifically for the Supergold.

Wiring for the TT was exactly as the Thorens manual describes with the chassis grounded to a single phono ground wire. The factory chose the right channel ground (green) but I used the left channel ground (blue) because Mission ground the 774 along the left channel ground wire. My Supergold has 3 pins so I just tucked the right channel ground out of the way. So TT, arm and cartridge all share a common ground down the left channel ground lead to the Quad 33 preamp. There is no separate ground wire and the mains supply to the TT is a simple 2 core sans earth as originally designed.

The result? Absolutely no hum at all and no additional hum when touching the tonearm

Clearly, in my other setup (1210/Jelco TK850s) I had got the grounding completely wrong. It probably doesn't help that there are multiple boxes connected to the 25RSLS preamp so multiple opportunities for grounding issues.

I need to revisit the 1210 grounding. I've just acquired another 1210 in totally unmodded condition sans arm so we will see but I just thought it worth flagging up that Decca's don't have to hum.