I just went out to my garage to check mine.....found 3 had the hollow triangle, one solid triangle (front). The solid triangle had a different mark in the one corner - it said "General" and I assume "Motors" underneath but its not casted clearly. The solid triangle one has a slightly thinner ridge on the front of the bezel around the lens opening. My car though low miles is not a survivor. Rear quarters are original, front fenders were NOS installed in 1978. Very possible the one front marker was replaced at some point or they could just be from different suppliers and both original. Tried to download pics, but having trouble with it. Joe check the thickness of the ridge the front see if you see a difference.
Jim