Chuck Berry was a guitar player. He wrote and recorded such songs as Johnny B. Goode, Maybeline, My Ding-a-ling, Rock and Roll Music, and many others popular in the 1950's. The horn you are referring to is a Chu Berry. Leon "Chu" Berry was a famous tenor player of the time. The Chu Berry horns, or New Wonder Series II, were great horns. They have a warm, dark sound, good intonation, and they play very easy. You are correct that the serial number prefix "M" indicates 1969; however, "M" was also used by Conn to signify saxophone, and the "M" appeared in the serial numbers of the New Wonder Series II and early "M Series" saxophones. The differences are obvious to most collectors. Most of these earlier horns are silver plated. Under the serial number there is an L to indicate low pitch. By 1969, low pitch had been adopted as the standard and there were no longer any high pitched horns being produced. The earlier horns also had a patent number on them which corresponds with the rolled tone holes. The 1969 horns have no patent number. There are numerous other differences; like the split bell keys, fingernail G# key, and other less obvious features.