Yahoo maps shows that zip-code for 5000 Longmont Dr., Houston, TX is 77056 not 77027 as addressed in memo #1.
The two post offices appear to be less than two miles from each other. The 77056 post office is west (outside) of Loop 610 and south of the Galleria. The 77027 is east of Loop 610 (inside). As I recall the Galleria was built in the early '70s. Someone should check the plaque on the post office for 77056 to find out when it was built.
Someone needs to go to the main library in downtown Houston and check Polk's City Directories (archives) for the years 1971 and 1972 re the Longmont address. Also check pertinent names while at it.