I've been communicating with an audio engineer named Brad Banks, who came across an HP-616A and the military equivalent, model TS-403B/U. Both signal generators cover 1.8 to 4 GHz, using pulse, c.w., or FM modulation, and have precision attenuators. He wants them to go to a good home - available for pick-up in Brooklyn NY.
The HP-616A sold for $1,950 in the -hp- 1953 catalog, the price of an automobile at the time. It uses a Klystron oscillator tube in a tunable cavity.
The units appear complete, but are untested. Photos are available.
If interested, please contact banksbrad at hotmail.com (not me).
Thanks, Ed Knobloch