The Scott Specialized combines the embossed envelopes, wrappers (and letter sheets together). Look at U293.
Add: The 1879 date refers to U.S. Patent 217,155 issued on July 1, 1879 to Lebbeus H. Rogers of New York City for his "Combined Envelope and Letter-Sheet". Here is the illustration page of that patent. The descriptive claims fill another page and a half of details. It took a few years for the USPOD to decide to produce these.