What Harmonic Balancer is this?

Dragonsteel said

OK, here is what's confusing, the block has casting numbers of a 71' 383, Was there ever an externally balanced 383?

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Yes a 1971 383 automatic could have came with a cast crank and be externally balanced. I've owned one. On another note later 400's with a standard trans came with a steel crank and be internally ballanced. Same thing, I have one!