What does this bit do.

[Downhilldai]

It's too near the axis of rotation to be a balance weight.
Once it's in the water you'll hopefully forget all about it. Get the prop bag on quick, before you spot something else

[Swifty]

Originally Posted by jyasaki

My guess would be that it is a block for a locking star type washer

I would concure with jyasaki, although I would have thought there would have been 1 the other side to balance it.
Interesting.