I've not seen your genre of nut before. So I'll just answer your Q. 2, Eric.
nuts on stainless steel
prop shafts are usually (often? mostly?) brass.
The brass is usually a manganese bronze, but that's a brass.
Reasons for the brass prop nuts:
1. so the prop nuts are not cathodes to your bronze propeller
(i.e. you'd rather sacrifice your prop nuts to protect your prop than vice versa) especially if your "bronze" propeller is also really a brass (as many are).
2. so the prop nuts do not gall or cold-weld themselves to the threads of your stainless steel
prop shaft. If you used stainless steel
nuts on a ss shaft, the tightening torque could lead to galling (cold welding). Brass nuts on stainless steel
threads are slippery and smooth and so do not cold weld, even when tightened. And brass is even more slippery on ss than bronze nuts.
A shaft anode protects the nuts, the prop, and the shaft.