"It took Congress about six weeks to pass the bill known as the 'U.S. Revenue War Bill of 1898' which was entitled 'An Act to Provide Ways and Means to Meet War Expenditures and for Other Purposes,' effective July 1, 1898. One provision of the new law required that all proprietary medicines be taxed and a stamp affixed to each box or bottle as evidence the tax had been paid.
"Meanwhile the Bureau of Engraving and Printing in Washington had anticipated the emergency and begun work on the dies, rolls, plates, etc., from which were to come the "Battleship" Documentary and Proprietary revenue stamps. However, it was very soon evident the stamps could not be printed and distributed by the effective date. Even provisional overprints of the postage stamps then current would not suffice to bridge the gap.
"In their dilemma eight medicine manufacturers doing business in St. Louis turned to the local Collector of Internal Revenue. He evidently succeeded in having the Department authorize stickers to be used in lieu of stamps during the early part of July, 1898...."
In the Morton Dean Joyce sale, June 4-5, 1991, provisional stamps of nine St. Louis companies were listed, plus one from Macon, Georgia. The Scott Specialized now lists these, with ten Saint Louis companies represented, plus the one from Macon.
Almost all of these stamps can be seen here, courtesy of Hermann Ivester: Saint Louis Provisional Proprietary Stamps.
One of the denominations of the stamps used by Antikamnia, with a first day cancel.
The Lambert Pharmacal Company was another of the companies with authorized provisional stamps.
Jno. T. Milliken & Company used two denominations of provisional stamps.
You can find the provisional stamps of all of the currently known users here.