Some of the symbols were cut into the visible face of the stone, but some were hidden in the interface between stones. So the marks were not all intended to be seen or read by the inhabitants of the palaces, though they were seen by the builders. According to the discussion that follows Hood's paper, the marks are not writing at all, but they look like it to me, for reasons adduced below. Iit seems likely from the different kinds of variation on basic forms that the symbols were more than unique arbitrary patterns. Indeed, some of the symbols appear also in Linear A and Linear B, which definitely are writing.
Arrowheads seem to be a common addition to a basic sign.
It appears also on characters on the little Knossos bar and the little black tablet shown below. However, the only one we saw on a vertical stone face in situ was the second from the left in the first row of the Malia set. Here is an approximately life-sized copy I drew from a small black bar from Knossos , which contains a triangle with a curved spike coming out of the top. Two more examples of similar writing in the Heraklion Museum. At left is my drawing of a small black tablet in the Heraklion museum.