From London Fire Brigade site: "The two numbers on the plate show the size of the water main and the approximate distance of the hydrant. In addition to marker plates, all hydrants are displayed on a computer in the cab of every London fire engine so that the nearest hydrant can be located while the crew are on route to a fire call." This sign, near Westminster Abbey, is still in imperial measurements and therefore is probably older than 1975, when they started changing signs to metric. If this were a metric sign the top number would be much larger since it would represent millimeters and not inches. I've seen a couple of older signs that seem to have a "D" coming out of the right vertical bar of the "H." I don't know what it means. I don't know what it means.