On foggy roads in north India, it is difficult to see vehicles on the road. So lights were used to indicate the status of big vehicles like trucks. Two lit bulbs on left and right corners (illuminating Horn Please) meant honk to get a pass. And to indicate you can overtake, the driver could light one bulb in the middle (illuminating a OK) to indicate its ok to take over. Eventually this system was scrapped but the tradition to paint "Horn OK please" remained.
(my god the theory is falling apart in my own head as I retell it).