I made gear knobs for a Deutz D15 and other small tractors with ZF A4 gear box (e. g. Holder, Porsche, Kramer). The Z-knob is for “Zapfwelle” (PTO - power take off), the G-Knob for the “Getriebeuntersetzung” (gear reduction). I heated the levers and melted the knobs onto them, it holds perfectly.