Why not go father back and contend that they copied the propeller from the Alphonse Pénaud helicopter toy that they played with as children in 1878 or so?
The propellers of Pénaud were a bit different.
See: L'Aeronaute, January 1872,
L'Aéronaute (Paris)
for a mere $25, you can get a CD-ROM set with over 2 million pages of history, including 16,100 pages of Wright Brothers papers
Library of Congress is free and has all the things about the Wright brothers that I can find on that CD.
see:
http://www.loc.gov/search/?q=wright+brothers