The Square D panelboard shown in the photo is type MO. The two-pole breakers were for the electric range and the hot water heater. The single-pole breakers were for lighting and receptacles. We had exactly this panel in my parents' home which was built in 1941. Type XO, which was available from both Square D and Cutler-Hammer was a commercial unit which I saw in gas stations. There were two vertical straight busses in the panel. The breakers had two clips in the back, one was live and one was a dummy. The electrician balanced the load by alterating the breakers -- breaker 1 to bus 1, breaker 2 to bus 2, breaker 3 to bus 1, etc. Thus the breakers operated on opposite directions. They had very limited life and could not be used for switching. It was a long time before the SWD-rated breakers appeared. Before then we used a row of toggle switches below the panel for switching lights at the pump islands, etc.
