The first printed stamped envelopes (issued July 1, 1853) and were produced by private contractors and sold at the cost of postage plus the cost of manufacture. With the exception of manila newspaper wrappers used from 1919 to 1934, watermarks have been mandatory for stamped-envelope paper since 1853. The watermarks usually changed with every four-year printing contract to help identify the envelope and paper manufacturers.
You might want to look for a watermark and authenticate the envelope....to help verify/establish the time/date.