Real or Re-Stamp???

It's BS.

What are those marks? A hand grinder?

you know, it really takes the fun out of it for car enthusiests to have to figure out if someone is trying to pull a scam. I guess eventually every car will be a Z-28 or Super Sport with "original" b.b.

What are those marks? A hand grinder?

Actually it could very possibly be real. GM did make mistakes from time to time, and they actually ground off the numbers and restamped over the area with the correct assembly date. They were called Grindouts by Al Grenning who has given lectures at Carlisle and Bloomington etc. on Stamp Pad odditys? I have several of these in my data base, although they are Corvette Pad Stamps and the ground out area was smaller.
Some of the TC Pad Experts would have to comment on this particular pad to voice their opinion as to Real or Not.

Its real.

I owned a Corvette with a grind out. The VIN and dates were not touched but the application code had been changed. They didn't try to hide anything, it appears the engine (1970 LS5) was stamped to go into a full size car and that suffex was ground out with a die grinder and the Corvette suffix was stamped. Original owner.

Agreed, it's real.
Gotta VIN to go with it (you can email / PM me if you want)?