Two very similliar symbols the one on the left is one of 3 stones by the side of the A351 just outside Rempstone. The Lichen cuts accross the carving on Rempstone Circle which proves the carving is old but I wouldn’t say contemporary with the circle.
Image credit: treehugger-uk
It's a bench mark. Won't be older than the 1800s anyway (Ordnance Survey used them to calculate spot-heights).