The one on mine is around 30mm but don't have the spanner to fit. this link may help ifyou can measure yours. http://www.sat.dundee.ac.uk/~psc/spanner_jaw.html
For what it's worth, I've got my D1 in bits and it's 1/2" Whitworth. This is about 0.82" AF, or about 21 mm. I always use a plug wrench on that nut, and the
I rode over to a paint mixing place and had them match something to a top yoke I had. Looked good in the shop. Got it back and it does indeed match that yoke