My 156 came with standard plastic Knob and i replaced it with Sport pack leather one.
Alfa workshp CD rom says to use a special tool to remove/refit but i did it without it.
The gear knob pulls off without screwing and the lever has a rib on the side facing the dash that allows the knob to seat in the proper position .
Be careful in removing the old knob as it takes some force!
When pushing it back on it will "give" when its reached it stop.
If your wondering what the lateral disc on the gear lever is for, i would guess its for locating the special tool. wink