Thoughts General Post Office Marker Posts?
General Post Office (GPO) marker posts were installed from the Victorian era until 1969, when the GPO ceased to exist. The marker posts often contain the royal cypher of the reigning of the time on them (like the post-boxes do) and were used to denote location of telegraph cables (for sending telegrams) and thus are no longer in use as people no longer send telegrams.
Starting with the reign of King George Ⅴ, the original cast iron GPO marker posts were replaced with a new concrete design. Thus, if you find a cast Iron GPO marker post, you know you have one which most likely over one-hundred years old. Such markers appear to be rarer than post-boxes, especially because many have been lost to the ravages of time.
Queen Victoria (VR) GPO marker post example: