OK, so it's warm out there. I am taking these readings by eye, but the setup is very stable - +/- a couple of counts once it settles. No surprise given the 150nV standard deviation.
With the bare copper - the last three digits switch from 480 to 490 if I reverse the connections at the meter (that's 48.0 to 49.0 microvolts)
With the nickel-over-brass banana jack - same result, once it settles. Doesn't take but a minute or two, it is warm out there (27 degrees). Off the bat, maybe 475 - 495 rather than 470 - 480.
I also did a quick zero test on the meter with a short, no surprise 1uV (10 counts) of offset (which matches the difference from reversing the cables, which is why you should always take two readings, one forwards and one reverse voltage.
Conclusion: the non-thermal connectors aren't making a significant difference if you keep air currents away, and you let them stabilze.
My garage is thermally variable, electrically noisy - but it is not drafty!
(I live in sight of some big TV towers - even my garage door opener will only work from about 25 feet from the opener, which means 10 feet from the door... so everything I do tends to be twisted pair).
Now, if you really want to see voltages move, try a nanovoltmeter.