3rd lead in the battery compartment

Ok so I have heard many people say that the d70 cannot accept any vertical grip and maintain some sort of shutter release capability or other controls ( control dial, etc) via leads. My only question is what is the 3rd hole on the inside of the battery compartment for? I see the two leads for the battery but then there is a third hole with metallic looking connections. Wouldn't this be what would be used for some sort of grip? If not, what is it and why is it there? Any insight into this would be appreciated.

Stephen Lareau