Early in 1939, as Ford and Ferguson rolled out the first 9N tractors, a tooling delay prevented Ford from having several of the stamped-steel components available for the assembly line. As a result, the first production run of about 700 9N tractors featured aluminum hoods, timing covers, steering columns, grilles and several other parts, which were then painted gray like the rest of the tractor before being sold. As these tractors continued their working life on the farm, many of the aluminum components became dented or mangled and by then, replacements were available from Ford parts suppliers. Replacement parts in steel, that is.
Image and info from Hemmings Blog http://blog.hemmings.com/index.php/2012/07/27/mecums-gone-farmin-auction-features-aluminum-hood-ford-9n/