I removed a factory AM radio from my 1966, C-20 that I bought as a parts truck, however it had the chrome knobs but not push buttons. Tuning was done by turning the right knob left or right. I have seen the push buttom units, however I do not know if they are factory or dealer installed options. I understand from my research that many of the options available on the '60-'66 trucks were dealer installed options vs. factory installed options as we are accustomed to today when buying a vehicle. There are reference materials available for a '66 that will identify what was available as a factory or dealer installed option. I believe it is called "Accessory List". I have seen them in a few of the aftermarket truck suppliers catalogs and on eBay. You should be able to identify this unit as a dealer installed option or as factory equipment. There are many Stovebolters that know this stuff inside and out. Woogeroo is one individual that comes to mind, as well as many others.