I think it wad a 2 year period, hetween lotto and call up.
Scroll down yo view the chart
https://www.sss.gov/About/History-And-Records/lotter1
“The last draft call was on December 7, 1972, and the authority to induct expired on June 30, 1973. The date of the last drawing for the lottery was on March 12, 1975. Registration with the Selective Service System was suspended on April 1, 1975, and registrant processing was suspended on January 27, 1976.”
Sss.gov
He could have enlisted in 1973, finished training and done a stint before the embassy disaster. Or could merely have served during that time, and been a “Vietnam Era” veteran.