The description certainly sounds like a bench mark, as Martin Roe has clearly indicated. Some of the oldest bench marks are the largest and sometimes found with a lead plug into the wall at the point of the arrow. Bench marks are also to be found on flat surfaces where a brass rivet at the point of the arrow determines the point of measurement above sea-level.
It is worth remembering that the height value of a bench mark displayed on a map is related to mean sea level at Liverpool before about 1914 and since then to Newlyn, in Cornwall. Just to complicate matters, some bench marks are rectangular cast brass plates in vertical surfaces for which a special bracket is required before accurate instrument readings can be taken from them; each trig. pillar usually has one.
If you should find a bench mark with the two outer edges of the arrow extended through the horizontal bar to form a cross, then it has been cancelled and no record of heights retained by the Ordnance Survey. Incidentally, all heights relate to the horizontal cut bar, or bench. The original purpose of the arrow was to show that the mark itself was Government property!
Any further queries on this subject to the O.S. please in Southampton; my service of 32 years as a cartographic surveyor is now but a memory.
Simon.
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