Jay - Without being able to see what you see, it's hard for those of use not up on the 503 engines to offer much help.
Can we assume (I dislike this word, but don't know how else to ask) that you've rotated the engine through the full circle and still not seen anything other than the "...barely discernible lines and lettering..."???
Can you get a picture of the lines and letters and post it for a look see? It might help someone (perhaps one of our GMC experienced folks) to offer some good advice or suggestions.
