The shaft ( blue part ) has a flat surface at the other end, where the small grey pin connect the green body to the shaft. I guess you can see it clearly in this view:
My first question is, why we do even need to give the surface such a flat shape?
I think maybe it's easier to bore a hole, but I'm far from being sure.
And is there any other flat surface on the other side ( opposite side ) or there is only one flat side ?