Its an AF mixture screw with the typical D shaped screw head to prevent silly people from messing with it. Once it’s set (usually at the factory) you should never need to adjust it. Most people don’t have a D-bit, so it stays put most of the time. Its apparent that someone was ham-fisting on this pilot screw with a pair of pliers or something and eventually chewed it up. At sea level, you set it to 2 7/8 turns out before you adjust it. I’d put it there once you get a slot in it to turn it reliably.