Discovery of the alcohol content? Many of the old elixir remedies were alcoholic and some still are. Alcohol is required to dissolve the vegetable ingredients. Genuine vanilla extract is alcoholic as are many other extracts.
Pinkham's Compound was and is one of many, so by itself was not responsible for labeling regulations.

The word 'compound' was used validly in its sense of mixture (composed of separate elements or ingredients, see Webster's), not the restricted chemical compound sense.