Yes? You put ink on the raised letters and press them to a surface and they leave a reversed impression. If you did that with these signs, the letters would be the right way around and the words would read from left to right, top to bottom.
Edit: actually the letters are the right way around on closer look, so it would still be fucked up as a stamp so that theory is wrong. Idfk what's happening here.
Yeah, and the ones that did are probably the bottom of the barrel.