That looks like it might be a Cutler Hammer Type BR 100A breaker feeding that panel. They come with a special hole in the breaker for a hold-down retainer screw when used to backfeed a panel. Did it look like the attached pics?
P.S. Backfeeding means that the feeders supply power to the panel through a typical breaker position, and not a factory installed main breaker at the top (typically on a panel that doesn’t have a main breaker, called an MLO panel).
It’s OK to do it that way, but the main breaker for the feeders in that panel needs a special hold-down retainer clip or screw from the breaker manufacturer to keep the breaker in position. Otherwise if the breaker is removed or comes out without shutting off the power from the source, the breaker will still be hot … :shock:
