I suggested at #175 that the "box" might be personal to Killian. And, possibly, the forger mistakenly took the box information from the previously released letter Bush sent to Killian.
You could be right but wonder why didn't he have a post office box on base -- I know at Wright-Patt, Kelly AFB, Tinker AFB, and everyplace else we have been, we had a post office box on base.