Earliest known 3970010 block?
 

Pretty certain that the earliest known 010 block is mid April of 1969. Correct?

Lots of misinformation out there. In fact, one "expert" claim it was used during the 68 model year in 327s.

Any definitive answer on the earliest known?

Earliest one I have seen was cast April 20 and assembled April 24.

Colvin's book says it came into production in late April or early May of 1969.

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I agree with the interchange info. That doesn't mean the 010 block was put into usage any earlier than April 1969. I have one in my 68 El Camino with all the 1968 bracketry.

010 blocks started in the end of March.

That interchange info is not helpful to the topic at hand nor is it accurate. As an example, I've seen 5 different blocks used in 69 Z28's - it shows 2. CBTN is more informative.