This is the posi drive screw that goes through the drum to hold it on. 2 of these are used per side, the 3rd screw that is covered by the drum holding the axle in is slightly shorter. We've found that suing an allen head for this is nice. When assembling, don't tighten the one under the drum too much before fitting these. You can still tighten it through a hole in the drum and tightening them a bit at a time in sucession seems to help get things lined up so the gasket seals a bit better.