As I tore apart my 73 convertible, I found a large # 3 9 written in what looks like soap stone on the back of the rear seat body panel (see picture). It was only visible when I removed the roof frame and vinyl well liner. Looks like it may have been put there during production. I have the Loise Eminger invoice and the car was sold in mid June of 1973 by Ford Marketing to Ford Marketing to a person identified by name on the invoice "H. Hartman", essentially an inter-company sale.
Has anyone seen markings like this before and knows what it means?
Thanks