The internals of the pivot are shown here.  The pile of washers (red arrow) just fits inside the eyes of the eyebolts and is kept there by the two big washers and the bolt and nut.  The pile of washers gives the eyebolts a sort of cylinder around which they can rotate smoothly.  A second pivot also exists because the the eyebolt that goes through the yard is only loosely bolted on so that the yard can rotate around the eyebolt.  When we actually rig the square sails the need for the two pivots will become evident.