Double VIN stamping on M Code 440 Engine Block

Exactly. They were always hand stamped. Hit the stamp with a hammer and if you don't think it was good enough you hit it again. Heck this triple stamp could have been the first try for a line worker or someone just filling in.
Something else to remember. These guys never for 1 minute thought it would matter, or anyone would ever again look at the number. Maybe for warranty reasons once in a blue moon. If they could have predicted the muscle car craze of the last 20 years, they would have bought a warehouse full of used cars for 500 bucks each back in the late 70's.