While moving from our old high school we found an old Sousa in the attic.  It has engraving on the bell identifying as Conn 20K USA.  The serial number is obviously 659347 which I think places the mfg date as 1957.  I was surprised that the horn is that old.  My question is regarding a prefix code at the serial number.  Offset from the serial number, preceding it is a "5" stamped.  I can not find any information on a "5" prefix, so I question if I am reading the serial right?