Just yesterday I read a article on those milk spots and the cause. It was quite a shocker how it has happened. Reading a book called volume 2 a guidebook to franklin and kennedy coins. In this book the author theorizes that as the proofs were being struck a mint worker gave them a dip in a ivory soap bath and then towel dried the coins and believes that occasionally a coin would have some soap on the planchet and when it struck it the soap became a part of the coin. In my opinion this is the most sound theory as to why this happens! Fun side note