Scott type U92 but Scott # U420, Die 1 ("UNITED" closer to the inner circle). Watermarks were used to 1) prevent counterfeiting, 2) to identify the paper manufacture, and 3) to identify regular or extra quality paper. Watermarks, just like the envelope contract, changed every four years. The overlapped U and S were in use as a watermark from 1915 through 1941. Year date across the top was in use from 1915 through 1933. Although I can't read the year date in the watermark it is probably 1925 or earlier based on the year in the CDS. Addition of the 1c stamp was probably to uprate the envelope to first class (frist class was 2c from 1919 through JUL 6, 1932).