so, dismounted the roof rack (once i located my stashed security pin hex wrench) and removed the rubber endcap of the wingrack. i tilted the open end of the wingrack, and lo and behold, part "A" — the threaded plate nut — slid right out into my hands. brilliant. here's the new wrinkle: the machine bolt securing the wingrack to its footing had not — in fact — worked it's way loose, but instead had sheered off completely. nice. next step, removing that, and getting a machine bolt the right size and length to match part "A." my brainstorm was to remove one of the other assemblies, so i could bring that to the shop in pursuit of a match. i have to give some props here, and say that the mounting assembly is really straightforward, even if they did use some slightly "specialized" hardware (specifically, the threaded plate nut).