I'll try to post some pictures in the hpagilent group later. There are quite a few pieces in this complicated system, but I've gotten it somewhat figured out, structurally.
The 70843B is normally hooked to a 70004A or equivalent display MF via two local control cables (I forget the name - something HP specific control interface), and the 70004A needs a synthesizer (I forget the model) module installed, which supplies the clock (100 MHz - 12 GHz). The source unit in the 70843B amplifies and squares up its clock input with a GaAs amplifier, and sends it to the front panel in true and inverted forms via two of the output amplifier modules.
The clock internally also goes to a band-switched variable phase shifter system for skewing over a wide range. Since the system knows and controls the clock rate, the bands and amount of phase shift can probably be worked out precisely to desired time delay or whatever else is needed. The clock from the input amplifier goes through an HP8768K coaxial relay which routes it to a selected phase shifter module - a 6-12 GHz variable (with three varicap sections), a 3-6 GHz (similar), or a <3 GHz block with apparently PIN-switched delay lines of 250 pSec, 500 pSec, and 1 nSec, plus one varicap variable section. There is also a straight through mode that bypasses all the shifters. A second HP8768K selects the appropriate output and sends it to another GaAs amplifier, then that becomes the internal clock for data pattern generation.
The RX unit has the exact same kind of phase shifter assembly, but hooked up a little differently. The input amplifier clock outs are not used, just terminated, and its version of its delayed clock is used for decoding the data stream.
The TX and RX units each have a block full of small board sub-modules (about a dozen) that do the sub-rate multiplexing (TX) or de-multiplexing (RX). I have been able to ID some of the parts, which are mostly GaAs IC source-coupled logic drivers or switches, made by NTT, circa 1990s. I'll list them all here for reference, once I figure out all the types and quantities. These run at the top speeds involved, and apparently the rest of the stuff runs at one-quarter or one-eighth the clock rate.
I'll have more info later, and maybe some pictures too.