Steve filled me in on the ECL code thing. I asked my contact at the GM Heritage Center if he had heard of such a document, which he hadn't, but he found something that may be it. So when I was in MI in my yearly pilgrimage to the Pure Stock Drags, I stopped by to check out the docs. Yes, this stuff will fill in the blanks for some info that Chevy people are dying to know, but it all needs translating. For some stats, they may correspond to a body code (like L78 Caprices:
https://impalas.org/1965caprice/) which makes things easier.
I'm sure Steve can explain it even better.