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on the other side the long #, When the so called "thin wall" blocks came out somewhere in the '75 MY they changed the 1st # from a 3 to a 4. That's the best I can offer.
Which actually turned into a falasy. Many blocks were independently tested and found many of the later blocks with thicker castings than the early ones. This was on both big and small blocks. Only sure way too tell is to have them sonically tested. 
Clean it, if it's Dirty. Oil it, if it Squeaks.
But: Don't fix it, if it Works!
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