Nope, the horn button isn't original.
When I first got the car 20 years ago, the first thing I replaced was the black ball shifter knob with a walnut aftermarket. I looked at that ugly rubber/plastic horn center and felt I had to do something about it. The horn center on 71 really was cheap looking if you ask me.
I liked the walnut so much, I decided to make a new horn center. Spent a saturday afternoon and traced the old one to make a new one out of Walnut. Tapered the sides and did a lot of sanding! I want to try again with another design someday. I like the walnut finish, but want a little more detail on the edges. Maybe some sort of dark border.
BTW: It's hard to see what it truly looks like in the photo - it's really a beautiful finish that matches the shifter perfectly. It looks thinner in real life too - the photo makes it look a little strange.