There are two different types of 701 carts - the one you mention with the empty chamber and the small area with a sponge and there is a version with a full sponge that takes up the entire area of the cart. What you have is the starter cart, the trial size, that comes with the printer. Just go...
I work at an ink store in a college town, and two scientists from the university just came in and asked me if these could be refilled, and I told them I'd do some research on it and call them back. The cartridges go in the following integrator...