[Jonesy]

That is an urban myth. The last 3 numbers of the casting were typically cast in both front (area under timing cover) and rear (area covered by the bellhousing) of block. 3970010 have the 010 and 020 because they are last three numbers of blocks in that casting timeframe. The 3970010 and 3970020 blocks.

You will also see this on small blocks made throughout the 60's.
1967 3892657 blocks have 657 and 944. (3892657 and 3896944)
Some 1965 3782870 blocks have 870 and 721. The 3790721 was the predecessor.
So it was a foundry thing to do this.


Its just a coincidence that the 010 and 020 blocks ended in those numbers.