My ´66 Charger and ´67 Coronet both have this distinctive "O" letter on the left side of the inside of the trunk (see photo). It is marked with the anti-rust substance used during factory assembly. My guess is that the auto workers responsible for rust proving did this up on finishing their work and to "O" is a short for OK.
Do you have other ideas?

Ragnar in Iceland

I don't see an "o".... I see a profile of a wolf with it's mouth open, and snarling. 🐺

Litmus test.....

I have never seen an "O" but have seen other writings. Check https://www.hamtramck-historical.com/ for examples. ,

Looks to me like where the bucket or can of that black stuff to the left was set down while applying it??

Good suggestion, followed by a swipe of a rag to get the big blobs off before sending down the line.

that's exactly what I was thinking, looks like they wiped the drips with the brush.

I've never seen anything like that in any Mopars from 1957 models up to 1974 , but I am a west coast guy out where we don't have to deal with rust and rust prevention

I wonder if the car was sold new in Iceland? Could it be something done before transit, or by dealer there?