Keithley, as well as Keysight, use copper contacts (i.e. tellurium copper, as they are lated/milled), without gold flash. These are custom Lemo connectors, not available in the Lemo standard catalog. The 181 and 182 uses standard connectors, MIL type, but these meters are also lower resolution/accuracy, perfectly fine here.
The chinese connectors mentioned look very nice, teflon used also for good insulation. It would be worth while testing them. And despite the fact that I hate the audio foolery too, I would think they are nice metrology connectors, but do not contribute anything to audio quality.