In the Haynes manual they are called ' rotocaps'.... page 90.

I do not see them discussed in the 617.95 FSM... but the picture on page 03.8-345/4 F2 shows it numbered item # 69.

They are only on the exhaust valves...and they are the first thing above the valve guides.
Each time the valve is activated it rotates it slightly.. this keeps the valve and the seat worn and cooled more evenly and adds lots of life to a valve... some cars have them on both the exhaust and intake...
I do not see them in the exploded drawings in the Performance Products catalog so they may be Dealer items only... they would be a replace ' on principle' item just like the springs in my book... can't cost much and do lots of good.