Not all that many horns had tags after 1928. Stewart Warner and Ames had stampings on the motor support ring. The EA Labs units had markings on the rear of the motor cover. The GIM units had different characteristics than the others and also had a stamping on the area around the ring but all of the different stampings can be faint if the horn has ever been sanded and repainted.
new--your horn is a Sparton. It probably is a rectangular tag with two rivets. The number you reported on it is a code for its production date, July 1930.