1974 September 22
St. Louis Cardinals vs Chicago Cubs at Busch Stadium (II)
Bob Gibson pitching, Brock #113, Riot vs Chicago (bench-clearing brawl against Cubs, starts at 2:34 of the movie)
Box score and play-by-play (including brawl details!) from Retrosheet.org
8mm movie (digitized to HD in 2019) by Michael W. Friedlander
KMOX Radio web site article from September 2021, i.e., 47 years later. (This appeared in my Facebook 'news feed' on 1 Nov 2021.)
Also, this great article about the same game from The Athletic (linked from the KMOX article), titled "What we remember, what we forget, and the brawl that beat back time".
The latter article points out how rare footage is of major league games from that era, which makes my dad's home movie all the more interesting and poignant.