For all 356 and 912 --- This is the special lock nut for the valve adjusting screw on the end of the rocker arms. Thread is 8 x 1,00 and it's 13 mm across the flats.

Tip: If this nut seems to thread too tightly over the rocker adjuster, the adjuster threads may have been distorted from over-tightening. Here's a link to a new adjuster screw: 369 05 243