644-23-220 Clutch Cable Adjusting Nut, 6mm. Fits 356 356A 356B T5 and 356B T6 (early).

For 356A  thru 356B T-5  models – This is the adjustment sleeve at the pedal end of the clutch cable.  It goes up against the cable anchor -- 644-23-219   --  and has a M-6 thread.  A normal M-6 hex nut locks it in place.