Likely not glass beads, but shot blasted or shot peined. Glass or media is not aggressive enough to do that much damage.
I'd bet the hole in the deck was someone's attempt to make sure he got his block back from the machine shop. I used a couple of letter stamps in an inconspicuous place on my heads when rebuilding in the 70's, just to make sure I got back what I took in.
Regards,
Steve