(You're gonna hate this) but it depends! I am going to assume you are not talking about replacing the internal tonearm, wiring itself. And you are just going to solder the unterminated end of the 501 to a terminal strip under the TT? If that is correct, then you DO NOT connect the shield to anything at the TT end -- you want it to float. Just hook up the L and R, plus and minus leads to their respective places on the terminal strip so that they connect properly to the cartridge.
There should also be a fifth wire in the 501 set that is separate from the 2 signal interconnects. At the RCA end, it has a spade fitting that connects to the ground lug of the phono preamp. The other end usually connects to the fifth pin of a DIN plug to provide grounding for the tonearm wand and turntable chassis. You will need to find a place on the Thorens to make this connection ;-) although there may already be a ground wire on the Thorens just hanging there, so connect it to that.
There should also be a fifth wire in the 501 set that is separate from the 2 signal interconnects. At the RCA end, it has a spade fitting that connects to the ground lug of the phono preamp. The other end usually connects to the fifth pin of a DIN plug to provide grounding for the tonearm wand and turntable chassis. You will need to find a place on the Thorens to make this connection ;-) although there may already be a ground wire on the Thorens just hanging there, so connect it to that.