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:

1. E1 = 1968 to serial # DT1-002850

2. E2 = (Domestic) was only used in Japan

3. E3 = 1968 # DT1-002851 through 1969 # DT1-015829

4. E4 = 1969 # DT1-015830 through 1971

Additionally, if you see a star symbol stamped on the flange. That represents the Japanese Government has inspected it. They may not have inspected every single carburetor, but the star inspection shows that they have looked at the carburetor batch and approved it. The fuel system is pretty important, as it has potential to start a fire.