If you look closely, at times you can see Kubrick operating a film camera.
The final sequence (the Walter Cartier vs. Bobby James bout) was filmed live on April 17, 1950 by Stanley Kubrick and Alexander Singer.
The film premiered as part of RKO-Pathé's "This Is America" series and opened at the Paramount Theater in New York on April 26, 1951 as a short subject in a program featuring Mon passé défendu (1951). Headlining in a live stage show was Frank Sinatra, which also featured an appearance by blond bombshell Dagmar.