I ran into a variation of this SqD panel in a service relocation and upgrade I did some time back. The panel was the first that SqD offered for general residential use, starting in the late '30s. The breakers were factory installed. One selected from a list of breaker configurations. . .and you made the wiring fit to that.
One of the unique features is that the breakers and hot buses were mounted on an internal pan that was not as deep as the cabinet. The pan was hinged on the left side, front edge, allowing the entire bus/breaker assembly to swing forward and out.
All the neutrals were laid on the back of the cabinet (while the bus/breaker pan was out of the way) and then the hot wires were hooked up.
A SqD rep sent me copies of the advertising from the late '30s championing this design by showing a svelt lass in party dress using one finger resetting a breaker with a caption to the effect, "Never search for a fuse again!"
This system (shown in Rennsport's photos) is what the XO system came from. The big advancement with the XO was the ability for the installer to configure the panel at the site from boxes of individual breakers that plugged onto the bus.
