AFAIK all EMIMs were open back, and no EMITs were. I know that Michael Alan Marks (the guy on eBay who sells lots of Infinity stuff at high prices) once published an article on converting two EMITs to a single dipolar EMIT, so if you see an open back EMIT, that's probably what you've got. But that procedure never made sense to me, if you want a bipolar EMIT, just wire up another one out of phase and install it on the back, just like Infinity did on so many models.