Yep, they earth through the dynamo frame. F stands for field which is the windings around the (fixed) yokes. The regulator adjusts the current through the field coils which alters the magnetic strength and thus the amout of juice generated by the spinning commutator. The commutator output is connected to D via the brushes. If my recoolection is correct, one brush is earthed to the dynamo frame and the other goes to ‘D’. The connection to the field winding should go to ‘F’.
Hope that helps,
Keith
