Stamped, But Un-Used, Circa 1887 Cover

I love stuff like this. No postal markings anywhere on this cover. No contents. The cover is sealed (maybe just from sitting over time) and has not been sliced sliced open.

I would think the two most likely possibilities would be that it is either a SASE that was never used for some reason such as the company wanting to use their own stationery, or that someone had an ongoing correspondence with someone and prepared in advance a bunch of stamped and addressed envelopes to use as they needed them to save a bit of time from doing them one by one. The fact that it is sealed does not really mean anything since as you say humidity over the years can wind up sealing such things.