Spotted these in a dry stone wall while out for a toddle. Looked them up when I got home, seems they could be 1 of 2 things.
1. Masons mark, like a signature. Used as a way of distinguishing which masons did what so they got paid accordingly at the end of the week.
2. Orientation markings to instruct the builder where to put the piece of worked stone in relation to the building.
I’ve no idea which they are! Interesting though. Makes you wonder where the stone came from and how it came to be where it ended up, built into a dry stone wall in semi rural Huddersfield.