Some cars got them some didn't, if I did not find markings on my car I would not put something on it that "might have been there",or just to make it look like someone elses's car.
PLUS the markings were painted over during the black out process. they would have only been visible to someone who studied the firewall very closely.
The cars with the marks over the black out paint are in all likely hood wrong, (Unless there is photo documentation showing the markings over the black out paint.)