This coupler is for the model S200 burner (or R200 burner), it couples the burner motor to the pump. There are two sized ends: one is 1/4" for the pump shaft the other is 1/2" for the motor shaft. It also comes with a small prop fan that helps straighten the direction of the combustion air.
The S700 burner has a heavy duty coupling that provides the same function.
