The connectors used by Keithley with that input were custom-made with low-thermal-EMF metals. However, I have had reasonable results with the standard mil-style connector, but that particular one is hard to find.
Not sure, why I thought they were pure copper connectors, but they're not. The manual clearly states that they are gold-flashed copper-tellurium (much like e.g. the connectors on a HP419A null meter).
I didn't get a hold on the original connectors (my instrument came, as so many others, w/o connectors -- who's hording the connectors?), but had to make-do with the Amphenol MS3106A16-11P, which is apparently meant for military application using high currents and has connectors with a silvery surface (tin?). One I used as a short to evaluate my Keithley 181 nanovoltmeter (about -130nV offset and ca. 20nVpp noise with filter enabled, well within spec). A 2nd one I connected to a cable meant for microphones (shield + pair of copper strands) to which I crimped copper fork terminals (from the stash I got from TiN a while back). When I bring those terminals in contact, I get something like
I'm quite happy with the noise (the excursion in the beginning is, I believe, due to range switching when starting data collection, the excursion towards the end is probably due to RFI), but is this ~1uV offset to be expected (it doesn't seem to go down, so it's not some EMF due to initial temperature difference, e.g. due to handling)?
Edit:
It might not be due to handling, but the 1uV offset seems to be EMF after all. If I touch the Amphenol connector (which is already quite warm -- seems to be the best outlet for excess heat from the meter, perhaps a design weakness) for some 20s, the offset races some 200nV in about the same time with a little lag. This particular connector had solder cups at the end of the pins, crimping those didn't seem practical. For the shortening plug I managed to bend the short sturdy copper wire so that it'll touch both pins before soldering, when soldering the microphone cable I didn't quit manage that it seems.