OK, Many of us have seen those letters stamped on the firewall, online and at car shows. They are usually neatly placed next to each other as if they're a 'word'. That is the product of 'restorations'. Look closely at the three letters (PTB) that are seen on my firewall. Those are original. And it's how the line-inspectors applied them. The P for 'paint' inspector was suppose to check the car for quality and then stamp it to certify he did so. Then the T 'trim' person and the B 'body' person. (not in that order) Too bad they didn't do their job. You should have seen the rust glob on my drivers fender right on the top, over the tire. And it was PAINTED over. BRAND NEW!
The holes seen were for holding various electronic ignition modules over the years. The last time was my MSD. The only problem with that is that spot on the firewall is HOT. That's why they call it a FIREWALL. The MSD died so I had it repaired and moved it to the front of the radiator support.