Following Steve Jelf's thread on Champion X plugs and the Champion ads raised a question in my mind. If the Champion X was the official spark plug since 1911, what was used prior?
"Soot proof" plugs were used during 1909, pictures in this thread: http://www.mtfca.com/discus/messages/29/48808.html?1203951468
Soot Proof and Splitdorf were used in the 1908 - 1911 period, and perhaps others. Ford typically had more than one supplier of spark plugs through most of the period of T production.