Hey there, I still have to check my notes at home but I’m pretty sure that the sire in question that Sharon May-Davis has linked the C6-C7 malformation back to was Northern Dancer. I learnt this at a talk she gave, she didn’t tell us exactly but did drop enough clues that the more TB knowledgable people figured it out. 
Thank you for going through the trouble to find that info for me! I appreciate it!
