ECN Forum Forums Photo Discussion Forums Violation Photo Forum Square D / Cutler Hammer

I'm sure it is.
Why do you think it is a problem?

How else would the busses be powered? Backfed mains are fine as long as a hold down bracket is used.

That panel predated the requirement for a hold down on plug in backfed breakers, as 384-16 (f) showed up for the 1st time in the 1990 NEC.

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.