Looks like "Pcs" under the "Nat". That reinforcement may have been the top piece in a container, with the supplier and the number of pieces noted on it. The fender outer "skin" came from Bristol Road Stamping, but that long inner reinforcement was welded to the fender "skin" at the car assembly plant (see the '68 Assembly Manual, UPC 11-13, sheet A3).
I have "GR" written on the inside of my lower fender extensions. My car is an Olympic Gold RS Z/28. Any idea what this means? I would post a picture but the file is to large.