35 mm film strips
From Just Solve the File Format Problem
The film strip was an instructional multimedia format commonly used in schools from the 1940s to the 1980s. A filmstrip is a spooled roll of 35 mm positive film with images arranged in sequential order. The frame size is smaller than normal 35 mm film, such that two image frames of a filmstrip take up the same amount of space as a single 35mm frame.
Film strips were designed for use with simultaneous audio recordings. These audio recordings would include tones or signals used (either manually by the film strip projector operator or electronically by the projector itself) to indicate when the next image on the film strip should be projected.
References
Filmstrip at Wikipedia
