To facilitate collection of taxes on distilled spirits, the Treasury Dept. of Internal Revenue divided the country up into a series of tax districts. These were organized according to State so, for example, the State of Alabama constituted the "Alabama tax district".
States that were home to numerous distilleries (Kentucky, North Carolina, Pennsylvania, and Tennessee, among others) were divided into numbered sub-districts (e.g. "7 th district of Kentucky").
Each distillery within a tax district was then assigned a number, such that the combination of tax district and registered distillery number ( "RD #" ) provided the government with a unique identifier for the purposes of record keeping. The familiar Detrick Distillery, for example, was RD #60 of the 10th District of Ohio.
The implementation of Prohibition in 1920 forced all but a few distilleries to cease operations. The handful that remained continued producing spirits for medicinal and industrial use. Many of the tax districts were collapsed and the distilleries reassigned RD numbers.