Lousy photo of a Brandauer No. 17F attached.
Having thought about these peculiar nibs for a while, I can see two reasons for making them. Firstly, they're quite attractive, and presumably confer the same benefits (to some degree) as the Mordan pattern cranked nibs without looking like a miniature battle axe. And secondly, it's a fiendishly clever thing to do, and one should never underestimate the professional pride of toolmakers. There was a traditional rivalry between the makers of raising and cutting tools, so whilst the familiar elbow nib is a tour de force in terms of the former, that curved slit is an equally impressive feat by the cutters.
Incidentally, Brandauer had a reputation for slitting. In the Paris Exhibition of 1867 they showed "pens of ordinary size with six distinct slits, all contained within the compass of a medium point, each slit being perfectly smooth, and all seven members of the point being uniform in thickness". (Charles Hibbs, from an article on the pen trade published in 1883).