Charter boles are indentations found in the boundary walls of Scottish buildings constructed between the 15th and 17th centuries; they were used to denote ownership and responsibility for repair. They are similar to bee boles but smaller in size.
"in Scotland a single charter bole, one foot square or less, was often built into a wall to indicate its ownership".
"charter bole: a rectangular recess used to house charter documents defining ownership of adjoining properties."
It shows that the "wall belongs to the property on this side of it".