It was called a Panzer III Schienen-Kattefahrzeug SK1. There is a pretty good article at the bottom of the page of this link, including plans and a front view.
http://axistanksworldwarii.devhub.co...pecial/page-3/ If you do a Google search for Panzer III SK1, you'll see a 1/35 model of it that someone built (no pictures of the underside, unfortunately). -Mike