Does anyone remember the name of a continuous movie format used in schools in the '70s?

If I recall correctly, the film was in a plastic, transparent cartridge no bigger than 5"x5"x2". It was squared off on the end that plugged into the projector (where the film ran, I suppose), and rounded at the oppsosite end, and there was a skewed circular protrusion at an angle to the main front-back axis, presumably to accommodate the continuous winding.

Tx
Bongo