I was questioning the same thing! Wondering if it were some sort of reverse negative photo post or something. I would highly doubt the sign is a reproduction, but the reverse intrigues me. It certainly never would have been hung at a station, S-V wouldn't have allowed it to go up if it is indeed backwards. Texlite Dallas did make those red border signs in the very early 30's, no question about that.
It looks a bit rusty/toasty, so I wonder if it was a defective sign that might have been scrapped and buried at one time? Just a thought.
Always looking for Mobil items I don't already have!