The CH 100+ amp breakers used to take four pole spaces (two adjacent two-pole positions). On the right side, the breaker clipped along the rail. On the left side, there was a screw/clip that was installed to lock it down. If you look very close, you'll see the locking screw on the left side rail.
Living in a tiny tiny town, I get to see every breaker that'll fit in every box. :P
A SquareD Homeline breaker will fit in ALMOST every other manufacturer's panel, but others' breakers won't fit in the homeline panel, due to a slightly different shape of the mounting rail.
QO breakers usually aren't interchangeable with anything.
