In most cases, one- or two-digit numbers are actually mold numbers that indicate the specific bottle mold or section in an automatic bottle machine. If numerous molds were identical, each one received its own number. Base numbers also indicate bottle styles or shapes, manufacturing dates, or factory location codes.

How do you tell the date of a bottle?

Most bottle manufacturers molded the year into the glass at the bottom of the bottle in 2-digit format. Youll often find it in the lower right portion when looking at the bottom (some dates are much easier to distinguish than others). This will usually tell you–within a year or two–when the whiskey was bottled.