Very unusual to find a military cover with documentary revenue stamps used illegally as postage. Three strikes of a "POSTAL EXPRESS SERVICE" cancel tying the stamps, along with a violet AEF (American Expeditionary Forces) censor handstamp. I'm not 100% certain of the cancel date, but it appears to be February 4, 1918. There are no markings on the reverse.
I'm not versed on addressing conventions of the era, but if the address at top left is a return address, then I interpret this as the person mailed the cover to himself, meaning that the cover is philatelic.
Is that an accurate assessment?
Still, very interesting regardless.