This information was provided by Enduronut.
These numbers can be found, stamped into the metal, located on the carburetor flange.
Yamaha Carb code Break-down information:
E1 = 1968 to serial # DT1-002850
E2 = (Domestic) was only used in Japan
E3 = 1968 # DT1-002851 through 1969 # DT1-015829
E4 = 1969 # DT1-015830 through 1971