[rit]

In 1991, Ashland Chemical Company, A Division of Ashland Oil, Incorporated, P. O. Box 34567, Houston

I do, however, question the use of the number in early 1970s.

[Eagle of Liberty]

Some ZIP code history:

http://www.usps.com/history/his2_75.htm
http://zip4.usps.com/zip4/welcome.htm

Entering "34567" in the USPS ZIP code lookup gives me the following result:

The ZIP Code you entered could not be found in our database. Please confirm the ZIP Code and try again.
If you are certain the ZIP Code you entered is valid, please send an email to incsc@email.usps.gov describing the problem.

[Barney59]

How about the 'TEXAS' in the address line, it should have been abbreviated in the old style, I would assume... Tex? or did the two letter state abbreviations start in 70?

[Bonaparte]

The US Post Office could confirm/deny this.

[Bonaparte]

The US Post Office could confirm/deny this.

[TeleStraightShooter]

In 1991, Ashland Chemical Company, A Division of Ashland Oil, Incorporated, P. O. Box 34567, Houston

Does anyone here realize that Ashland Chemical Company and HALLIBURTON share the same Lawyers?!

</sarcasm>

[njmaugbill]

From USPS.gov. The "zip Code Lookup"section:
PO BOX 34567

HOUSTON TX  77234-4567
Carrier Route:  B006
County:  HARRIS
Delivery Point:  67
Check Digit:  2

Q: Do ZIP Codes™ Change for PO Boxes?
A: "ZIP Codes for PO Boxes usually do not change, though the ZIP+4® will likely include a code in the plus four that designates the actual PO Box. For further information regarding this, contact the local Post Office™ will have that information."

This means that the PO BOX number in this case is a combination of the last 2 digits of the zip code, and the Zip 4. In this case the documents show that the PO box is the same number as the zip 4.

The only problem is that there was not a zip 4 in 1971 or 1972.