Some do, some don t - it all depends if there is a road level that is important to the cable depth; this one does not have a horizontal marker on top of the Ordnance arrow, some do - I have written to the GPO archives and they will be giving more of an idea. This one looks to be fairly old (and from a chip shop).
not quite 18 its 17....but I could be wrong.....have to go and check in the morning now....but you are right with the road the number was for the xtra points anyway.... funny I thought you d get this one! ::15: