I am not sure what is this, I just bought it to complete my Columbian series of this design.
However, it looks that it was used in Germany... and that Postsecretair (Postmaster?) W. Brell, from Steglitz (part of Berlin today?) used it to send it to himself?? There is a lot of notes overall the cover, so, maybe he explained what was his purpose to do such a thing with an US stamped envelope.

