Tundra Restaurant Supply is always happy to help customers find the parts they require.
I can't be certain that this Red Plastic Burner Valve Knob will fit your Southbend 400 series range. That said, I was able to actually find the replacement valve knob for the 400 series ranges. It's not red (it's black) but it's the exact piece you need.
I've included the knob below for your convenience.