The button X of the Xbox 360 doesn't work on Linux.
The button X of the Xbox 360 (the button number 03) doesn't work on Openspace3D application compiled for Linux.
With the Xbox 360 controller (Afterglow) the X button is recognized but you have to weigh it hard for it to recognize it and sometimes it does not recognize the button. I don't know if it's a bug in openspace3d or a driver problem. However, I tested the controller on Linux in another program and the X button works.