My book says Daimler-Benz and Skoda and tons.
Encyclopedia of German Tanks of WW2 by Chamberlain and Doyle
German Panzer IIIN als Schienen-Ketten Fahrzeug SK1
(Panzer IIIN with railway suspension added)
23 tons (because that's what it says right here...look!, look!...yup, tons)
1942