The first watermark that appeared on U.S. postage stamps was the "double-line" USPS, for "United States Postage Stamp", first used in 1895 on the "First Bureau Issues".
Although the lettering appears reversed on the stamps above, it would read normally when viewed from the front. As it turns out this is only one of eight possible orientations when viewed from the back. The watermarks were always applied parallel to the top edge of the paper and are never found in any orientation other than the eight described.
Double-line watermarks appear on the following U.S. stamps:
The "First Bureaus" - Scott 264-284
The Trans-Mississippis - Scott 285-293
The Pan Americans - Scott 294-299
The "Second Bureau" 1902 Issues - Scott 300-322
The Louisiana Purchase Commemoratives - Scott 323-327
The Jamestown Exposition -Scott 328-330
The Washington Franklins - Scott 331-366, 422-423, 460 and 519
The 1909 commemoratives - Scott 367-373
The following Special Delivery stamps - Scott E5-E7
The following Postage Due stamps - Scott J38-J44
The following Officials - Scott O121-O123
The following Newspaper stamps - Scott PR114-PR125
As well as on some revenue stamps.