USUALLY...the large number ("39" in this case) is the sequential home game number for that season. The Cardinals played their 39th home game in 1979 on June 29th against the Phillies. On August 13, 1979, Brock hit #3,000 against the Cubs in the Cardinals' 59th home game of the season.
You can check any number of websites to determine game-by-game results for any team; I like
www.baseball-reference.com (under "schedule & results").
It's hard to imagine that game #39 would ever jive with an August 13th date, so I'm skeptical of the authenticity based on this information.
One other thought...I checked an old seating cart for the stadium (from a 1985 pocket schedule); none of the gates or sections were numbered "39". Perhaps the Cardinals used that large number for some other purpose, but I doubt it. Unless someone else on the board can solve the mystery, I think your ticket is in question.