See the little front-panel switch circuit board? It's got two red wires coming in from the mains, and orange-green-orange-green going out to the transformer. As you said, for 230V (as configured) the two primary windings are in series; for 120V* you put them in parallel. (You'd need to test the exact color coding, since it's possible that one primary winding is orange and the other is green — or that each winding is orange on one end and green on the other.) Quite possibly, that's exactly what the unused connector footprint on the switch PCB does. (Either that, or the second connector is for the input voltage switch, and when that's not used, jumpers are installed. My HY3005 clone is like that.)
*It's not 110V and hasn't been for nearly 100 years!