I have seen some cars that have black markings on the front of their valve covers, usually numbers and or a letter.

What would be the correct markings for a 69 440 4-speed ?
I saw a very nice Daytona 440 4 speed car that had a 915 on the front of the pass side valve cover and on the drivers side there was a O on the front, would that be correct for my GTX ?

Also can someone tell me what they mean ? I am guessing maybe engine type and trans type ? did all of these cars have those markings or only some and what type of a marker would a person use to put them on, I would think over time they would maybe burn off.

Thanks