Mustang Markings

You probably need to be more specific about exactly which marks and also let us know which assembly plant and time period you're referring to.

On a Dearborn car around that period, typically might see a rotation number somewhere on the front, usually a 3 digit number written in white/red/yellow on the bottom edge of the core support or possibly on the driver side of the radiator support. Chassis markings on the passenger side were typically marked before the car was painted and obscured once paint applied to the engine bay.

Additionally, you might find some torque/checks in various places, like a yellow or white 'X'. Sometimes red marks can be found on suspension pieces to indicate a part has been torqued.

Other than that, you might find paint inspection stamps (the baseball diamond variety) on the fenders and hood.

If you don't have any information from your own car to go by, I would suggest you start building a library of cars close in production at Dearborn to yours. You might be able to find some consistencies that give you a better idea what was common practice.

I would recommend that you hold off on applying any markings for now, unless you are duplicating something that is known to be on the car originally. Then, add some later as you learn more.