When a Mason becomes a Master Mason he is presented with the an apron that has blue rosettes in place of those "medallions" (called levels), two on the bottom and one on the top of the apron.
The man who owned this apron obviously had this apron from then and when he became Master of the Lodge, he removed those rosettes and replaced them with the levels, which are a symbol of the Worshipful Master or a Past Worshipful Master.