DD,
Good advise from my two fellow trackers...however
before you file this one away as a curiosity, or just a masons mark, SS was correct in his assessment about masons marks and getting paid but that was for a cut and dressed stone! not a unquarried wall!
So try something just to make sure,... take your compass with you, next time you go there.
1. Are you looking North when you are looking at this wall ?
2. if you are then this sign is pointing at the Cardinal Directions.
3. Doubt that a masons mark would be so Oriented.
4.These two signs are both Individual classic signs,
5. the square is a well used sign in Spanish, KGC and others .
6 the diamond is also a critical sign.
7.both together should attract much attention.