Did D.B. Cooper leave a calling card?

After the hijacking, agents from the FBI and newspaper editors received countless letters from anonymous writers claiming credit for the hijacking. Some letters had a comic tone ("Yabba Dabba Do! Admit it, I've won! You've lost!") and others were braggy: "I have succeeded in pulling off one of the most successful, talked about crimes of today." One letter though had a different and depressed tone, almost suicidal, a condition that was consistent with many hijackers from the era. It reads:

 

I am no modern-day Robin Hood. Unfortunately I do have only 14 months to live. My life had been one of hate, turmoil, hunger, and more hate.

Check out the clip to the story, printed in the Los Angeles Times. The report also notes the letter came with a code, a mysterious sequence of letters and numbers: "7698QA2753." Any code crackers out there?