By the date of manufacture. Conn moved their production of student level horns from IN to AZ (not Mexico...yet) during the 1960 to 1961 time frame. The likelihood is that it was made at the newly acquired, BEST MANUFACTURING PLANT in Nogales AZ since it dates to this period. In the early 1970's, production moved to Mexico, and the horns were so marked. These are the horn often derogatorily nicknamed Mexi-Conns, that garner so little respect due to decrepit manufacturing and quality control. I wouldn't worry too much about the likelihood that it is not an Elkhart made horn. It is a student model, and the early ones (including yours) are still better than the later versions.