I'm not much help, but if it would be of any assistance, I could take a photo of my horn and send it to you so you'd have an example to look for....the instruction manual for the car says the horn was provided by "Moto Meter Gauge & Equipment Corp., Toledo, O."..... I could look and see if there's anything on the horn that would give a clue as to part number or whatever.
I believe the original horn is round and about 5 or 6 inches in diameter. Mine mounted on the head with a simple bracket to a head bolt.
I believe the original was not to effective so many owners added another on the firewall.
Gary