Depending on your bike and the derailleur manufacturer, there is a specific shape bracket used for mounting it with the hanger (it is included" with the derailleur at purchase) The problem most mountain bike enthusiasts have is that, due to rough terrain conditions, this hanger gets damaged. This is a simple and elegant solution. One can make enough so that if the hanger does get damaged on a ride, it can be changed fast and will do the job, at least until a metal one is bought. the M4 thread is not included, as it is easy to make with a regular tap, or directly with the crew, but the 1mm stpe M10... well that's not as common and it's safer to just have it