Some problems that arise trying to use period publications to verify it, such as shop or owners manuals, is that they tended to reuse pictures from prior years if there wasn't a mechanical change. In the case of the '56, the shop manual is a supplement to the '55, and only covers the mechanical changes. The only interior shots in the owners manual look to reuse the same picture found in the '55. I have looked at the advertising brochures on the Old Car Manual Project, unfortunately the one interior picture is somewhat fuzzy, can't really make out what button it has.
I don't believe that the "Art Deco" button is Passenger car, I've never seen one in any Chevrolets I've ever been in. I have picked a few '56 trucks for parts, but honestly never paid much attention to the horn button. Either I simply didn't look, or it was not present on the truck.
