I think the ranks of Fähnrich and Oberfähnrich are analogous (in the RAF and British Army - not so sure about the RN) to the modern day Officer Cadet.
These are trainees, enlisted and paid as private soldiers, but may understudy a regular officer until they are themselves fully commissioned. They are given the honorary title of OCdt and certain privileges like use of the officers' mess. From reading Iron Coffins by Herbert Werner, his duties aboard his first boat as a Fähnrich sounds pretty close to that ... and being the most junior, his officers dumped all their post-patrol paperwork on him the moment they reached port!
