Found an HP3457A for 95$ on Ebay "As-is", the unit had a problem which was described as "Adrs 22 Failed".
I had a suspicion and bought the unit. On checking the unit, it gave immediately FAILED after showing the GPIB address.
After some online research and checking with the scope, the unit had no communication between the digital board and the analog board, this is a bi-directional serial signal of app. 190 Kbit between processors on each board. At power on, the digital board probes the analog board but there was no signal on the analog side of the opto-isolator. Conclusion: opto-isolator broken.
The problem is that this opto-isolator is unobtainable. The HP3457A analog board is fully isolated from chassis ground and digital board, there for the opto-isolators and the powering by separate transformer windings.
After quite some prototyping/experimenting I found a good "drop-in"-ish replacement, with good isolation voltage and decided to replace both opto-couplers. The replacement of the original KT3172/HP#1990-1075 (A2 U509 and U510) is the SMT opto-isolator FOD-8160. This one can be glued upside-down on top of the "opto-gap" (see picture) and then the output can be wired 1:1 (top view) and for the LED-side cross-wise (top view).
The unit works great after that (last calibration 2014).


Schematic:

(correction still to be done on above circuit: +/- input symbols of output Opto-Coupler OpAmps are reversed, no impact to the above)