From what I have heard they weren't the best castings so I guess a lot of them were scrapped as unusable. I have seen one which had been planed so much that the spark plug threads came into the face.
Pretty serious reclamation work going on here.
Last year I got one of the reproduction heads from UK and it always brings out a yarn or two about the legends of the time who raced with one or had triple carbs etc. I really enjoy these stories.