According to: http://tinyurl.com/6xsqu 1966 1ST Position S (California) J (Jackson Center, OH.) 2nd Position I (International Model) O (Overlander Model) 3rd & 4th Position Length 5th Position Model Year 6, 7, 8th Position Sequential Production Model. (Over 500 Double Bed, Under 500 Twin bed. Dick Martin http://home.earthlink.net/~DriveAbout