It would be interesting to find out. It may be that someone bought repro housings that were so marked, even though it makes no difference to the solid state regulator.
Also seems hard to believe that, given the lousy accuracy of these gauges anyway, that anyone worried about orientation affecting the readings. The heating coil is literally wound around the bimetallic strip and the case is sealed, so the difference would only be whether the weight of the strip pushed it closer to, or farther away from, closing the contacts.