Reading that pdf manual was an education for me too. I have finally figured out why the grip was removable on the original 645. It was meant to be a remote control! Honestly, that was one of the great mysteries of that camera in my mind until now. And then with this remote battery pack I had assumed that the idea was just to have more battery power. But the manual makes it clear that the primary application was extremely cold weather. You take the batteries out of your grip and put them in the remote pack and then keep that warm in a pocket somewhere so they last longer in the cold. Brilliant!
Sorry if this was old news to you all... I'm always happy when I learn something new about an old camera!