Hi Lucy, Welcome to the forum and thanks for your recent contributions!
Here is a link for the benefit of others - to my Federal Law Forbids... page.
The "R" on your decanter stands for "Rectifier" which is a company which blended /bottled liquor into consumer bottles from bulk containers. A "D" - for distiller - was the manufacturer who actually produced the spirits. One would think that some distillers also bottled as well - I have no idea how that case was handled.
Note that the original bottle in this thread does not have FEDERAL LAW FORBIDS...embossing so the R in that context probably does not mean the same thing.
Unfortunately, there is no comprehensive information source for all the base markings of 20th century bottles. As you might expect, each manufacturer marked things in their own way.