>what is this hub kit?

from what I can understand, it is a hub, splined to fit the
prop shaft on the id, and with an irregular shape that fits into
the prop itself, in a way so it can't turn - mayby oversimplifying,
but like prop has a square hole that square hub fits into.

So props can be universal, fit different motors with just a different
hub. This hub incorporates some sort of material that serves the
function of the rubber bushing I'm used to seeing press-fitted into
the center of a prop. Clear as mud?:-)