You've got nothing to worry about until the black vans and helicopters show up!:shocked
Looks like a Rebuilder ID tag, or Restamp some call it - usually from a bigger rebuilder. Like a national engine rebuilder with many shops, they'll put their own id tag on it so they can find out whats been done to the motor in the past by them.
I've got one on my '88 351, from a shop in Alaska.
And if anyone from those black vans asks, my name is Rusty Shackelford :toothless