Here is an original power adaptor from Zenith. With many of their portable radios, Zenith added an outlet to accommodate the radio to be played on AC current. This allowed the radio to be played without having to maintain the complement of batteries that were otherwise required to operate the set.
The output of this adaptor is 9 volts, which means that if the radio took six 1.5 volt batteries, that this would work. This adaptor will work with the Royal 790M Navigator, the Royal 820 or 76 and the Royal 880 just to name a few. It will not work with the Royal 7000 Transoceanic radios.