My reserve unit uses a PO box.
There may be other reasons to question these documents, but the PO box isn't one of them.
Just to add to the information stack -- the current address of the 111th is 14657 Sneider St ... which is located on Ellington Field. This is also listed on the official website ---> http://www.ang.af.mil/units/angsites.asp?state=TX ...
Now I know that things change over the years, but I thought I would throw that out there.