Whiskey was often labeled as - and sometimes even embossed - "For Medicinal Purposes Only" as early as the mid-19th century - long before National Prohibition took effect in January of 1920 (Wilson & Wilson 1968).
To be fair, ethyl alcohol was (and is) one of the better preservatives for products intended for internal consumption or external use.
The famous (or infamous depending on perspective) Anti-Saloon League was primary force doggedly pursing the move towards the banning of alcohol and one of the first the successful single-interest pressure groups in the U. National Prohibition, however, was already the law of the land through Congressional passage - over a presidential veto - of the National Prohibition Act (aka the Volstead Act) on October 28th, 1919 which took effect immediately, although existing stocks could be sold through the January 16th, 1920 date.
There were, of course, various exceptions allowed under the law for "medicinal" products containing alcohol as well as sacramental wines (Okrent 2010).
Search google with that phrase for a webpage that lists many of the permit numbers assigned and used by many glass companies.
Many whiskey bottles are seen with “D-numbers” on the bottom which are distillery identification codes.
Date codes are often seen, especially on soda bottles from the 1930s to date, and many of these codes are embossed on the base of the bottle, placed to the right of the glassmaker’s logo. These date codes are not always obvious, or easy to distinguish from mold numbers.
It also depends on exactly which glass company produced the container, as all firms do not use the same system of markings.
However, there are definitive trends in shapes that mark a bottle as very likely to have been used primarily or originally as a container for high alcohol spirits intended for internal consumption, "medicinal" or otherwise.
In many cases (especially within just the last few years, writing this as of 2013), mold data information is now preserved through the use of small embossed “dots”, “bumps” or raised periods arranged horizontally around the lower heel of the container.
More information on this invention (which is rather involved!
) and how it works can be found by doing an internet search (Google, Bing, Yahoo or other search engine), using the keyword search terms “EP 0256804 B1 ” , “code reader”, and “Emhart”.
Liquor of all types - bourbon, rye, gin, cognac, scotch, etc.