Posted By:
JimBMaybe nobody liked Sullivan and when they got one, they threw them away. 
Just kidding. It is a great question, but I have no idea what the answer would be.
JimB
P.S. Interestingly, Peter Calderon said that when those were lined up with the E93 sheets, they seemed like they were once all one sheet. So it seems E93s and E97s, at least for some period (maybe only the proof run since these are blank backed) were printed together.