http://sphotos-a.xx.fbcdn.net/hphotos-ash3/75579_503721709666147_953371547_n.jpg
It looks like an attachment designed to provide a flat surface against a cylindrical object like a bar or pipe. The hole is where a bolt would pass through.
If there’s a name for this I’m not aware of what it is.
Like** lieu **said. It might be called a bushing. I’m sure there’s a name for it.
What they said. Also, you sometimes get these in pairs- flat faces mated, for joining two round components together.
Not, as far as I know, used for strange grid balls.
Seems to be called a curved washer by some, although others also use that term to describe something without a flat face
It is acoved spacer.