Perhaps a bit late, but it might be of interest to others who stumble across above. I found also a hard-to-read operational manual, but then also the service manual (which also includes the schematics) in much better quality at
http://exodus.poly.edu/~kurt/manuals/manuals/HP%20Agilent/HP%206186C%20Operating%20&%20Service.pdf -- Ah, strike that. There's only the simplified schematic, the detailed one (figure 7.4) is missing. Which is a pity, as the error amplifiers are built from discrete transistors and I'd like to have a close look on how that's done.
How's the supply working out for you? Do you got pictures? Use cases?