They're assembly line markings. The 704 is most likely the rotation number, which was the number your car was assigned the day of it's production. The 8 could mean any number of things. My 65 Dearborn had the same 8, but it was written on the outside of the inner fender at the battery box where yours is on the radiator support at the battery. It may be battery related, electrical related, I'm not sure. Anyway, it is interesting to find original grease pencil markings on your car, whether it's numbers like you've found or X marks where bolts have been tightened, etc.