I think just about every SR 400 block I've run across up to this point was ground and stamped - some just the last digit, some a couple and some all digits. Both of my 1970 RAIV SR blocks have just the last digit ground and stamped, and I've seen a few with the entire 9799915 ground and stamped. And several factory 'numbers' 70 RAIV blocks I've seen had the last digit ground and stamped.
I wonder if this 'AS-CAST' SR block might have been sold over-the-counter as a 400 block with 4-bolt mains to someone who was just wanting to upgrade or purchase a high performance 400 in 1968? Or a drag-racer? Guess anything is possible.
I wasn't really into Pontiacs at that point (1968) as I was only in the 5th grade.....
A couple years later, I discovered loud, fast Pontiacs. And then it took me a few years to save enough money to start buying them.