Whatever it is, it would not be a practice of an assembly plant to create a stand out paint anomaly. That could have prevented the truck from selling. Markings, as stated, were usually chalklike marker. A paint dept worker would have to have a good reason to do that. Especially that big! Sure is nice paint for 67 years?
Conspiracy theories:
1. As you say "wiped" not with finger and don't see brush marks. Looks like paint stick width.
2. Looking at the ridges of the wipe and it's still green inside wipe trench, I can't believe factory paint was that thick when wet.
3. Inspector was saying >4 (greater than 4 mils thick). Was showing how thick it was.
Never got fixed.
4. Seems more shinny than rest of firewall.
5. May not be original paint in that area. Can't tell.
You, being a paint instructor, can decide if orig.
