What happens so many years/owners/machine shops ago is never easy to understand or explain. I've been building drivetrains for a pretty good while, and I've experienced good and less than reputable machine shops. I sold a verified, documented LS5 SS '70 Chevelle a couple of years back, that had the original drivetrain still on board, with the engine completely rebuilt by a reputable shop in B'ham, AL - sure enough, they decked the block and wiped the numbers, in 1983. I understand the then-owner was furious, and it's probably why I was able to buy a 56K mile beautiful car in 1992 for next to nothing (I found the build sheet on it 19 years after purchase, documenting the car). The shop that decked it blueprinted the motor, but apparently disregarded the engine stamp in preference to building a good engine. Durned thing still runs like a scalded dog -
Point being, circumstance outweighs fact some of the time - casting date coding is unusually early (my L78 original block is one month prior in casting to the build date), I have another L78 VIN coded block the same way, but that sample size does not define the range of dates by itself.
That's why I enjoy, and hopefully everyone else that posts here, the CRG. Ain't it fun ?