This is the moveable piece that tightens the NukaTap to the Short / Long Shank but black in colour.

If you are wanting to mix up your Kegerator / Draught System with a bit of black flair, this would be a great place to start if you already have Stainless Steel Shanks and don't want to spend money on an entire shank again.
It is important to note to be gentle with these black parts as scratches will show up more clearly compared to the Stainless Steel look versions.