This is an amplifier for measuring very low currents (~pA) by measuring the voltage across a big high-precision feedback resistor (RH2HVS, 1e+11 ohms). The resistor and this other silver component (KP F) are both inserted into sockets (not soldered) so they can be easily swapped out.
What is the silver thing, I am guessing a capacitor (making for a sensitive RC circuit) but the "KP F" lettering is not getting me anywhere. It looks like foil encased in glass. Any ideas what it is (and where I might be able to get more)?