Be careful analyzing the x axis when it pertains to MSB or MSC. Your Krugman book uses the quanity of pollution on the x axis which means the MSB is represented by users of the product or the first party which is manifested as the the producer. For more info, see Krugman or the PPT under module 74. On the other hand, if the x axis represents the quantity of pollution cleaned up or the amount of pollution removed then the MSB is controlled mostly by the third party and the MSC is controlled mostly by the producer.