Holycrap!!! That is about the most robust gauge mount I've ever seen! You could probably use it to tow vehicles.
I'd say that this is a homemade part and someone just added the patent plate. Please post pics of the bike. I'm sure we'd love to see what other unique items have been made for this bike. Any large cast iron pieces attached to it. I'm guessing it might have some farm implements bolted to it as well.