The MG 08/18 was developed at the very end of World War One as a lighter alternative to the MG 08/15. It used an air-cooled barrel, and between not needing water and having lighter parts it managed to be about 6 pounds less weight than the 08/15. Only a few hundred appear to have been produced before the end of the war, and they were not used by the German military during the Weimar era (the Mg 08/15 was). The 08/18 barrel jacket served as the basis for the MG34 barrel shroud, interestingly.
Thanks to Limex for giving me access to this very rare gun to film for you!