The dog nut lock tab for the MGA Twin Cam is a different part and works differently than the dog nut locktab on the pushrod engine.

The illustration in the Service Parts List appears to show an external bent tab which might engage the keyway in the pulley, but this is not so. The Twin Cam uses a one piece cast iron pulley. There is a small hole drilled in the face of the pulley near the edge of the locktab. The bent finger of the locktab will engage that hole, and one side of the locktab should be bent up against the dog nut after tightening.