Hah! These folks are the best! I was just kibitzing--the real work was done by many others, perhaps especially Howlin, who got this train rollin'! I saw the Jim Angle piece, and he mentioned the fonts and the "th" part of it--nothing about the oddly different signatures and the P.O. box 34567.
Britt Hume and Bryon York are all over it now on FNC
As far as I can tell, the "P.O. box 34567" bit is legit:
http://www.harriscountyhealth.com/pcd/ViolationNotices/1991HCPCDVNs.htm
The above is for some sort of pollution violators roster, circa 1991, after Ellington field had lost it's exclusive military status. There is a listing for Ashland Chemical on Old Galveston Road (aka Hwy 3), which is where Ellington is located.
It may be a P.O. Box that was used at Ellington's zip code during the 1970's.