I believe that is BUTTON1 – although I haven’t tested the ways of referring to it.
https://docs.oracle.com/javase/7/docs/api/java/awt/event/InputEvent.html
Note in particular:
int button = InputEvent.BUTTON1_MASK
int button = InputEvent.getMaskForButton(1);
int button = InputEvent.getMaskForButton(MouseEvent.BUTTON1);