Old GPO cable marker on Downshire Road. Would appear to date from sometime between 1910-36 (probably towards the earlier date given when the road was laid out).
This was placed by the General Post Office as a marker plate for their underground Cable network. Originally this would have had a plate giving the distance in feet and inches from post to the underground cable, but this has been removed. Often mistaken as bench marks, the arrow actually signifies that the GPO is a government department.
Any other information that I can add on this is welcomed - a quick search of this site would seem to suggest that examples such as this are rare; another in Lisburn is seen at J2463 : GPO cable marker, Lisburn.