It is nice to resume discussion after a while, but it would be better to  
start from where the argument was left. A few years ago the same question  
was raised and answers circulated through the HES list. I myself gave an  
explanation (and a reference) close to Malcom's and others' in the sense  
that Marshall's diagram (and, as I now discover, also Knight's) reflects the  
producer's point of view,  
best  
  
Tiziano Raffaelli