I've walked past this carved stone hundreds of times on my walks to and from work and have always thought I should take a picture of that. It always intrigues me, just sitting there beside the sidewalk, and reminds me of a hobo sign, though hobos didn't carve them into solid rock.
After doing a bit of digging it looks like a British Ordinance Survey benchmark. This makes sense as it lies next to the Garrison Grounds, on the bottom of Citadel Hill, the 18th-century British fort. Good to know!