From the F.T.C. website: "A registered identification number or RN is a number issued by the Federal Trade Commission, upon request, to a business residing in the U.S. that is engaged in the manufacture, importing, distribution, or sale of textile, wool, or fur products. Such businesses are not required to have RNs. They may, however, use the RN in place of a name on the label or tag that is required to be affixed to these products." Click on the RN Database - Search link to find information about the manufacturer/importer: http://www.ftc.gov/bcp/rn/
The Canadian equivalent is the CA Number administered by the Competition Bureau: http://cb-bc.gc.ca/epic/internet/inc.../cp01120e.html
You can find out interesting information from these numbers, such as the fact that the J. Press shirts made in Canada are made by The John Forsyth Shirt Company Ltd. of Cambridge, Ontario and the U.S. made button-downs are from Hathaway (old stock?).