I notice that the popular Bormioli Rocco stoppered glass bottles have numbers on the bottom. Some higher quality ones I have are numbered "18" and some more recently bought ones have the number "14". Does anyone know what these numbers mean?
I did some some more research and Ecnerwal is right, it is a mold identification number.
Each manufacturer numbers their molds kind of like a baseball team has unique numbers for each player. This number is placed in the center of the bottom of the bottle.