In fact, the letterhead must include the actual 'physical address' of the unit.
Don't aviation units in all services use the name of their airfield as their location? A squadron is a relatively small unit and I think it unlikely to have had multiple HQs. What AFB did the 111th fly out of? And wouldn't that,rather than a Houston PO Box, be their address?
The sequential numbers certainly look hokey. I can't speak for the Army or ARNG, but the Navy and Marine Corps went to PO Boxes for addresses in the mid 80s in compliance with a Department of Defense directive on postal addressing standards and are still using them today. One thing I learned in 23 years as a Marine is that you can rely upon the Pentagon to be factually incorrect at least 50% of the time. You can go to www.usmc.mil and search under directives for mailing addresses and it will give you the order (MCO5110.5D)from 1999 and the references in adobe.
I have to disagree. My unit, 2nd Bn, 121st Infantry, 48th Enhanced Separate Brigade, Mechanized, of the Georgia Army National Guard, uses PO Boxes. In fact, the PO Box for brigade headquarters was PO Box 4848.