The first watermark that appeared on U.S. stamps was the "double-line" USIR, for "United States Internal Revenue". This watermark was placed on paper stock used to make revenue and documentary stamps. It is included here because at least three issues of United States postage stamps were inadvertently printed on the USIR watermarked stock: The 6¢ and 8¢ stamps of the First Bureau Issues of 1895, Scott 271a and 272a respectively, and the $1 Prexie stamp of 1938, Scott 832b.
According to a Report of the Commissioner of Internal Revenue the watermarks were to be placed in such a manner that each stamp would have at least part of one or more letters visible and that each square inch of the paper the stamps were to be printed on would contain one letter. Thus the horizontal spacing from the middle of one letter to the next is approximately one inch, and the vertical spacing from the bottom of one letter to the bottom of the next is also approximately one inch.
It is thought by most authorities that the "U" and "S" watermarks of the normally watermarked "USPS" postage stamps are indistinguishable from their "USIR" counterparts, and that either a clear "I" or clear "R" is needed to fully authenticate the "USIR" variety.