Any markings? Nowadays you see a lot of small alternators built into the fan assembly, expecially on small diesel tractors and such. But I'm certain they didn't have such a thing back in the day, given the lack of control electronics and whatnot.
It is an authentic Ford Approved accessory to fit '42-48 Fords, released around 1946 and in the catalog. It had a zingy name like "Electromatic" or something in the ads. The kit can be ordered at your Ford dealer as 59A-8665, unless he is out of stock... It is controlled by a little thermo sensor that inserts into the lower radiator base the same way as the heater hookup in those years. I think, but I'm not sure, that the innards are not a moving clutch but an electro- magnetic sludge that solidifies when fed electricity. I've got one, and have posted my successful search for the sensor...which involved paranormal intervention, for sure!
From an old post of mine...I was hunting for a sensor, but the sensor was apparently hunting for me! "I used to have one lacking the essential sender unit...leading up to a true story of the weird and paranormal: Joblot used to allow me past the counter so I could spend hours hunting through their shelves for stuff I wanted (THAT was a serious privilege, poking through a vast heap of NOS Fordage gathered from all over the world!). One day, as I walked down an electrical shelf, a piece fell off the topmost rack and actually hit me on the head. I picked it up and found myself holding the sender I needed! I crawled up and went through the upper shelves to reunite it with its fan, but there was no fan...they had only exactly what I needed, and of course I bought it for a buck or two... "
How about a picture of that sensor in case something similar happens to me so I'll be able to identify it?
As I recall, it looked like a Bakelite mushroom. I think there is a good pic from a Ford flyer in the '46-48 book, "Closing the years of tradition." I suspect that the sensor could be replaced with a streetrod electric fan control...trust me, you'll never find another original! I just happened to meet one that was looking for me.