This is interesting to me, so I will give it a go. I have 32 copies of this stamp in my used USA stock. I laid them out all out on my desk, and was not sure. Then I flipped them all over, and was able to see differences. See my two scans below. All stamps are flipped over in the same position between the two scans. Not sure which row is blue-green to Scott, but I will guess bottom row? What do you think?