I have made some captures of the Sidewinder controller protocol on an old desktop PC with gaming port.

It starts a handshake using all buttons and at the end falls into an operation mode where it sends the button data in 5 chunks of 3 bits on Button lines 2, 3 and 4 being clocked by button B1 line .

If no initialization is performed the controller uses only the Button 2 line to transmit data and the Button 1 line to work as a clock