I'm gonna stick my neck out a little, and say it's quite possibly correct if the casting date is OK to the build. My 350/300 Z11 is a 388 block, I'm 99.999% on faith in it's originality, stamping is very similar to this one in font and appearance. I think someone took a sharp screwdriver to scrape the pad with to expose the stamping, not try and duplicate the broach marks - and I can't see any feed lines that would indicate block decking with a mill. All of the stripes appear to be hand motion, not machined feed lines, and erratic. One thing for sure - some doofus really butchered it.
If I can chop my way through the garage, I'll get a shot of my pad to compare it. Don't lose hope yet -
Regards,
Steve