On many bottles, a single-digit date code along with the diamond/oval/I mark may indicate the 1930s.From information compiled in Bill Lockhart’s article (link below) on Owens-Illinois’ date code markings, it appears that, on containers with this earliest trademark, if a single digit date code (such as “O” or “1” placed to the right of the logo) the chances are very good the bottle in question dates from the 1940s, especially the 1940-1947 period.
It also depends on exactly which glass company produced the container, as all firms do not use the same system of markings.
IMPORTANT NOTE: Many liquor bottles and flasks made by Owens-Illinois have a DIFFERENT mold code configuration on the base, as compared to the way the numbers are arranged on most other types of bottles they made.
Typically, it is marked with a number called a “Liquor Bottle Permit Number” followed by a dash and a second number which is the date code.
(If problems occur with the finished product, it can be easily ascertained which mold or mold section is at fault.) Numbers also serve other purposes, depending on the exact container and/or company being discussed. Some numbers (for instance, 3- or 4-digit numbers on the base of many British bottles) are catalog, inventory, style or design numbers assigned to a particular bottle shape.
Those numbers would serve to identify a particular bottle style, such as in communications/orders between the glass manufacturer and their customers ……is, the companies who ordered the bottles to package their products.