Choppers.
There is a very useful discussion about the various types of chopper in the article on DC microvoltmeter by D. Bollen in Wireless World May1967. He settles on a reed relay.
See also the manuals for the Marconi TF2603 RF millivoltmeter. The 1965 germanium transistor version uses a relay CB1711 as chopper. The 1977 Silicon version uses a relay chopper C1417 and the 1984 version uses FET switches (2N4416).
The 1965 version has a section upon testing the chopper relay.
I also looked up the Marconi TF1313 bridge. This uses photocell choppers, Clarex CL703CL, driven by the light from a neon relaxation oscillator. The method RadioWrangler suggests.
The Boonton 91C uses a chopper relay, Stevens Arnold CA12 or Bristol C1417-8.
Diodes.
The Boonton 91C uses two gold bonded germanium diodes, no details given. However the Marconi TF2603, with an almost identical circuit, uses in all versions two germanium diodes in the probe,type AEI CG91H. These became Mullard AAZ13 or GEC GEX66. Similar are Hughes HD1871 or STC DK21.
The Electronic Engineers Reference Book, by Hughes and Holland 1967, with abstract into
www.technicalscientific.com/techinfo.htm and quoted by Electrojumble.org in 2012, is an interesting discussion upon design practice used in Marconi TF2603 of heating the probe diodes resolve temperature dependancy and also of the noise level of the various types of chopper.
At one time. I worked upon the chopper relays used in Kent or Honeywell chart recorders for chemical plant control. Used Clare 3 pole change-over mercury wetted contacts, relay HG3A 1036, plugging into an 11 pin octal valve holder. Another one used a RadioSpares mercury wetted reed relay.
I suspect that mercury wetted reed relays are much more reliable than a standard reed relay, as the mercury will prevent contact bounce and possible arcing.
I have no details of these, but have the manuals and articles I mention for all the others if you cannot find them. Also some spare diodes AAZ13 or similar.
wme_bill