Strange stone

I have never been able to work out what the marking are or say but they appear to be more than mere masons marks. It sits at the base of a stone wall adjacent to a road near here I walk Shadow along and it always catches my eye, partly because it occasionally disappears from view when Ivy sometimes grows and covers it.
Nightshift no.4 and it's been a really busy week, much more so than usual. I like being busy but full on with no let up for a full set of shifts doesn't half take it out of you!