I've seen pics of service replacement engines stamped SR then the six serial numbers of the car. Could be fake who knows?

I think the SR engines had the block serial number and the block code but the lower number were left off and then stamped on when installed in the car.

I'd say your original was taken out at sometime in the last 51 years and replaced with one from a different car. No big deal, just not numbers matching.
My cars Vin indicates it was a 6 cylinder car when made. It had a 428 in it when I bought it. Works for me.