For what it's worth, I've changed out my fair share of CH style resi panels, usually ater the main decided to self destruct. (see here for one of em). I think I also remember there was a metal bar that people would remove to get twin breakers inside the thing.

The supply house I typically use is a big Cutler Hammer supplier, but not for resi stuff... I'll ask how much just for the heck of it [Linked Image]