Came without mounting hardware maybe ?
I had one of those 6 screw/bolt walnut wheels with a horn button. Sold it with a car not installed.
It was given to me from a guy going out of business.
Did you goggle the name ? I believe I found a horn kit assembly at Speedway Motors.
Don't remember the manufacturer name of mine but but the horn/spline/collar/ kits actually work on several manufacture's wheels with the 6 screw/bolts.