"Geoff Miller" wrote
According to one of the photo captions, the stainless steel
prop reduces the top speed by 5-7 mph despite both props'
being 14.5" diameter and 21" pitch. Why would two props of
identicial specification have such different performance
characteristics because of one being steel and the other,
aluminum?

It's the other way around, the stainless is faster. I believe it's primarily
because the blades of the aluminum prop are much thicker (in order to get
sufficient strength from the weaker material) and this reduces its
efficiency.