The story goes, that the DC-9 wing was a barely modified DC-8 wing. The DC-8 had excellent stall properties, so the DC-9 was expected to show the same good qualities, but it did not. Eventually, it was discovered, that the pylons of the DC-8 were part of the good characteriscs, so what they did was to build a mock mini pylon on the DC-9 wing, and presto, there were the good stall properties again. The little 5 days stubble under a DC-9 wing therefore, is just a deteriorated DC-8 pylon. Or so the story goes...