I was looking at the fender tag for my GTX last night. I'm getting ready to dip it in a de-rusting solution and was gently just getting the dirt off of it. It's almost impossible to read the front of it, but you can read the back pretty well. It looks to me like there is a # 5 laying sideways punched on it. I'm not sure though. Maybe after it soaks it will be more clear.

I was also looking at a fender tag that I took off another RoadRunner that I used for parts (I am going to de-rust that one for a trial run before I do mine.) and that one has a P laying on it's side and is punched high-side on the back of the fender tag. I took some pics of it, but they are on my computer at home. I can post some later tonight.