Scot - That's a really interesting observation. I say this because another collector contacted me a few days ago with a scan of one of his cards with a similar mark on the back. He asked if I thought that his and the mark on the Schulte could have been from the factory just as your proposed. I told him that though they looked similar I didn't think either were done at the factory. I thought like you the likely explanation was they were similar stray marks added by a collector. I don't know what use these markings would be to the factory, but there's always an outside chance they could have been used for something. Here is an image of his cards mark and the Schulte mark.
