That 40 NEMA tag might have come with a multi-wattage M-400 or M-400A. GE always wires their lights for the lowest wattage so if it was a 250/400W HPS fixture, it would have come with a loose 40 NEMA tag so that if the light were to be wired for 400W, the 40 could be stuck on over the 25 that is already on the fixture.