I never looked it up but just assumed it was the U.S.S. Pueblo...
I agree with FReeper sit-rep. To the best of my knowledge this song was a tribute to the men of U.S.S. Pueblo.
That is a reasonable assumption, however there were 83 men on the Pueblo and it sailed out of Japan prior to capture.
As I understand it, the lyrics were not written with a historic incident in mind, and the number 73 refers to the keys on keyboard used when the song was written.