Joe -- Not particularly. The biggest difference is that the 20K series used a three position Selector switch for: Mono, Automatic, and Stereo Filter, with the last position being a forced (non-automatic) stereo position. Starting with the 30K units, they added a fourth position to this switch and labeled it "Stereo". This too is a forced stereo setting, but without the sub-channel filter engaged.
The 60K units converted from the three pole relay that switched the audio signals and Stereo Beacon (like mine), to the receiver style design using a diode matrix switching circuit to handle the audio switching, while a simple SPDT relay takes care of controlling both the Stereo Beacon lamp, and the audio matrix switching circuit.
There were other minor changes between the various versions, but these were the significant changes.
Dave