No way a unit would have a post office box when they were stationed at Ellington. The box may have been real but no way that was the address of the squadron. I would like to see any documents he sent the squadron with that address. AF would never have paid for a post office box and can assure you that the post office wouldn't have given them one in a different zip code area to boot.
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.