39 CFR § 221.7 - Postal Service emblem.
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.