1970 3963512 Block Strange One

[3macs1]

Just when you think you have a grip on the GM numbers bang you get hit with this. Probally 20 years ago I pulled an original 454 engine complete out of a 70 impala. I knew the entire history of the car since it was purchased in the family new until the day I sent it to the junk yard.
About a month ago someone finally convinced me to sell the 454 since it would make his 70 chevelle closer to original. We agreed on a price and off he went.
This motor was never apart. He called today to ask me if I knew this was a 3963512 4 bolt main block with a steel crank.Of course now he is looking for ls-6 heads, etc.
Has anyone ever heard of GM doing this. This engine was 100% oval port, 3963512 2 bolt 70 Impala as far as I was concerned.
Last motor I leave the pan on before sale that is for sure. Boy did he get a deal. I never even looked at the side for oil filler plugs, etc.