The metal boundary marker on the right is marked St P P (for St Pancras Parish) 1854. The large stone has E C carved into the other side of it, for Eton College, which owned this land until 1842 when it was given to the Crown for the creation of Primrose Hill park. On this side of the large stone is a benchmark, which usually denotes government property (possible added during WWI or WWII when Primrose HIll was used for army installations and anti-aircraft guns). I didn't see any marks on the smaller stone. I think this is the boundary of St Pancras Parish with the parish of St John at Hampstead, but I could be wrong about that.