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Once machines were used to blow milk bottles, the look of milk bottles changed. Early milk bottles were made using the Owens Automatic Bottle Machine which left a round scar on the bottom of the bottle. The Owens machine used suction to pull molten glass into a mold and the glass was then cut by a knife which left a scar mark. The Owen's suction scar, as it was called, is a rough off center mark.
This machine was widely used from circa to And, two digit numbers on the bottom of milk bottles from this era may represent the plant code or the year of production like 15 for Another bottle machine used a plunger to remove the glass bottle from its mold leaving a different type of round scar, the size of a penny, on the milk bottle.