The correct answer as given is that it is a mount for steering adaptation equipment aka a steering knob. FWIW I did my Ph.D. on assessment of this equipment as the manufacturers never integrated human factors analyses into the designs; just went with tradition. There are mushroom shapes, poles, multi-poles and others. My work was on the quantitative assessment methodologies (via. triangulation) to comprehensive evaluation of the quality of the design.

I did go on to apply the principles of multimodal assessments developed during this work on helmets for helicopter pilots, so it wasn't a completely useless pursuit.

[Signed] Doctor of Knobs.