340 Harmonic Balancer

What year? '68- early '72 is internally balanced. Late '72 &'73 is externally balanced. But not the same as externally balanced 360s.

The 340 cast crank is external balanced. Forged crank was internal. The cast crank damper only fits 340.
They switched to cast cranks with external balance on the 340 mid-1972. I know because mine was.
It may have even been early 72. I have seen more 72 cast cranks than forged. But, I dont think there is any official documentation to verify one way or the other. They just used forged cranks until they were gone.
April of 1972, if my piles of records are correct. They also changed the block color from hemi orange to blue.