Here are the facts.
1. Vendors don't do extensive research, they rely on best info they can get from customers, restorers, parts books and manuals, if they even go that far. By now most of their "mistakes" have been weeded out.
2. There is a Chevy "Art Deco" horn button of that era, it is shown in many photographs of old trucks and is sold, repro, as being around 56.
3. Part numbers can be published too early for a design change.
Whether a part number is found or not, it did and does exist.
4. It can be a Cameo. It can be some sort of option. It can be non-truck.(researched some, not found)
5. The WWW shows it on mostly 56 trucks and Cameos.
6. Documentation may be out there somewhere.
7. Best reasonable assumption: 1956, 1956.1, 1956.2, 1956.3 ....or Rev A in the parts book.
8. We have all given our best judgement, I don't claim any better knowledge than anyone else.
