This is the nut for the early 12A1215 rocker adjusting screw.
Use with 7/16" AF spanner.
Recommend: 8 x 6K654 per Mini.
For the tappet adjusting screw, see 12A1215.
For the later 1/2" spanner size, see AEG167 for the screw and NT605061 for the UNF nut.