Amiga 4 Joysticks adapter
This adapter will make it possible to connect 2 extra joysticks to the Amiga. This requires that the game is aware of this Multi-Joystick Extender in order to use it. The adapter is connected to the parallelport of the Amiga.
(To the 1st Joystick).
9 PIN D-SUB MALE to the 1st Joystick.
Source: [The Hardware Book]