Answered By: Regan Brumagen Last Updated: Jul 19, 2016 Views: 55
According to our curator for American glass, Jane Spillman, numbers engraved on the inside of the neck were used to match decanters with their stoppers. The stoppers and bottle necks were ground to fit exactly and then both were marked with the same number. This was an English custom which was also used in the States. So the bottle/decanter with a number on the neck means it once had a stopper which is missing now.