Here is a perfect example of how the E.III cowling appeared.
http://www.atpm.com/7.01/washington-...-louis-420.jpg
The swirled texture was achieved by using a hand-held power buffing wheel and (I assume) a rubbing compound. This was a popular treatment of natural metal cowlings in the teens and 20s. You can see the effect being created in the movie "Spirit of St. Louis" starring General Jimmy Stewart.