Many thanks for Philip Colston for procuring, scanning, and sharing these manuals ...
… for the General Radio 1806-A Electronic Voltmeter.
The first is the Revision A manual for the 1806-AR rackmount version of the meter. The circuit is the same as the non-rackmount version. Revision A applies to instruments manufactured before January 1965.
The second is the Revision B manual for instruments manufactured in January 1965 or later.
IET Labs hosts the Revision C of the manual in their archive of downloadable General Radio historic manuals. The schematic in Rev. C is identical to that of the Revision B manual.
Philip has a nice post on the Antique Radio Forum describing the design changes over the production lifetime of the GR 1806-A.
Update:
Philip has produced smaller versions of the files by converting from color to gray scale and reducing the resolution: