follow-up
I have never seen sand cast casting or date codes changed on a block. I have however seen dates machined and restamped from the factory on some 409 W block intakes and verified that it was legit.
I think I have even read that early Pontiacs had some factory machining of cast numbers for special SuperDuty applications. (blocks and intakes) I cant confirm though.
I personally have never seen this alteration on a Chevy block, factory or not.
I have seen decked blocks restamped- missing the factory broach marks, and getting the "4" wrong on the numbers (years ago when I was playing with vettes). My understanding now is that they got the stamps right and can even replicate the factory broach marks-Forgers are persistent.
In this case I am wondering if you are talking about the front pad.
Scorp