My dad had this in his safety deposit box. I'm failing to see any error. He seemed to think it might be a mule but I've found it surprisingly hard to research this online. Front plate number is H478. Rear is 507.
One other possibility is that the ink on the back came from stacking. In this case it's a wet ink transfer. Again probably a variance and not an error. The note would have to be effected more dramatically to be deemed an error.
Took this to the ANA show. A dealer immediately recognized as an offset printing and now that he pointed it out, it's very clear to see. I was just thrown by the "Reverse error" notation on the holder.