Hi! I need help in identifying which model my Yanagisawa alto SN 7751115 could be? It has a single 5 stamped to the left of the SN and logo. It has a high F# key, and single arm on the low C. The 7 digits confuse me. On a 7 digit horn the first two digits would indicate month, and the 3rd and 4th the year. Month 77.... Using the 6-digit logic, this would be an A5 from July 75 (month 7 in 75). The last 3 typically indicate some sort of production order. But here there are 4 digits!?! Any thoughts?