I recall seeing some HP8620 plug-ins over the years, that were labeled as Hughes, but none contained an oscillator as far as I could see. They had connectors on the front for control/drive cables that went to various external waveguide-mounted oscillators, which were presumably separated from the plug-ins and long gone.
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A quick look online for "hp8620 hughes" showed an example on ebay:
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I don't know who actually built them but they look like sort of commercial, low volume custom units, including the plug-in and the scale. I've never knowingly seen any of the oscillators. You can see on the back of the plug-in a line power in and out, which probably ran the power supply for the oscillator via the RF ON/OFF switch. You can also see that they were built from parts or sacrificed plug-ins, not custom made by HP - they would have looked much nicer if so.
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Ed
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