Found my dads original 1971 duster!

The viscous clutch drive allows the fan to 'freewheel' when it is not needed to cool the engine, which reduces parasitic drag on the crankshaft, freeing up power. I don't know about 15 hp, but it does help, I have used them on my Darts and been happy.
I definitely notice a difference when the fan drove engages on my highway tractor.

Your car may have been equipped with one from new.