Here's my other post on hpagilent from before:
"I've been doing some searching on the part number of the output modules 5086-7956. Got some hits including apparently used in some version of the 8590 series SAs. I couldn't find anything like it so far in documents on these SAs though. Maybe it's just a wide-band (100 MHz-12 GHz) amplifier, used in other gear too."
I've been studying the output module more, trying to trace out the small board attached to it. I opened up the module too. The circuit is very tiny stuff on sapphire substrates. Looks like three identical GaAs amplifier or logic chips cascaded. I need to set up a microscope to see more detail.
Live voltage measurements on the interface from the big board to the small board/module were confusing because in operation, I think the whole amplifier is biased up and down to get the right DC out, while its input is AC coupled. Without knowing what conditions the instrument defaults to, it's hard to tell what it's telling the module. Opening it up will help to tell at least what's what in there and the basic signal path and structure.
Will post pictures and rough schematics if I figure out enough to make some sense.