the photo = students graduating in gown and "mortarboard" caps

In the literature of Freemasonry, the story is told = that when the great cathedrals were being built, boys would carry mortar up the scaffolding to the stonemasons, way up on the wall.

In order to be able to use both hands for climbing, the mortar was carried in a "hod" - that flat board - fastened upon their head.

which infers the analogy that such graduates are good little 'hod carriers' serving Freemasonry