I had 757 on mine but it wasn't a stamp. Looked more like white crayon. Great big red five in the tub between the supports from the rear deck to the floor and a 010 in what looked like black grease pencil drivers side door jamb in the front bottom.
Different assembly plants did things differently. Dearborn, Metuchen, and San Jose (that last one was only the first couple years)... One used colored stickers, another used paint stamps. They all used crayon for major assemblies... Believe it or not, your best bet is to get a Mustang Club of America judging book. since Mustangs and Cougars came off the same lines at the same time. Mustang, Mustang, Mustang, Cougar, Mustang, Cougar,.... Bear in mind, there are assembly plant differences, and also shift differences! Oh, and parts supplier differences. It's enough to make you wanna tear your hair out!
I don't have the hair left for me to be tearing it out, so that's why I'm not into the concours restorations. Resto-mod for me!