Aaaah, my eyes!! It looks like a screw stuck in it, but it's actually a slit in the plastic. It is a drain plug, the van I'm working on is a deluxe microbus with a factory sunroof over the cargo area, the drain pipe for it goes under the roof, down inside the B pillar and that plug fits on the end of the pipe and sits in a hole in bottom of the pillar which is why the end is angled. I'd never seen one, not sure if it's a 'late' feature as I've never seen it on the few early westies I've worked on with sunroofs. Maybe they weren't included because they were made for Westphalia, so they left the drain pipes out. It's a weird design because that slit was blocked with rust which meant the bottom of the drain pipe rotted out as well.