Around the middle of 1964 major changes occurred. The info below is very useful for dating a Gibson guitar in cases where serial numbers were repeated or unclear.
1. Neck width at the Nut was reduced from 1 11/16 inches to 1 9/16 inches (you will see cases of 1 10/16 inches during this transition year.
2. Most Hardware moved from Nickel plated to Chrome, this included bridges, tuners and tailpieces.
3. Most serial numbers moved from 5 digits to 6 digits.
4. Kluson Tuners were stamped with 2 lines down the back instead of one line down the center of the back with the words Kluson Deluxe.
5. By 1965 the Stud Tailpiece on the Gibson Es-335, Es-345 and ES-355 were removed and replaced with a Trapeze tailpiece.
There is more information here on the our Guitar identification Page.