§ 221.7 Postal Service emblem.
Primary source
Verbatim text below is from the Electronic Code of Federal Regulations (eCFR), a public-domain U.S. government work. Always verify the current version with the eCFR before relying on it for any legal matter.
Full Text
The Postal Service emblem, which is identical with the seal, is registered as a trademark and service mark by the U.S. Patent Office. Except for the emblem on official stationery, the emblem must bear one of the following notations: “Reg. U.S. Pat. Off.”, “Registered in U.S. Patent Office”, or the letter R enclosed within a circle.
This is legal information, not legal advice. Laws vary by jurisdiction and change frequently. Always verify current law with official sources and consult a licensed attorney in your jurisdiction for advice on your specific situation.