Factory body marking

[runuts]

I had my 68 up on the lift looking inside the frame rail and noticed a green factory body marking. This was on the left side only at the front near the leaf spring attachment area. Has anyone seen this before and what does it mean?

[KurtS]

Looks like the stamp from the manufacturer of the flat steel stock before it was stamped and formed into the frame rails.
I've seen it on gas tanks before, not common to see it on other parts since they were prepped and painted.

[runuts]

Is the frame rail boxed or is that the underside of the body?