A leap of assumption: 1904, which was the first year for "the first commercially successful six" (according to Georgano.)
Btw, my collection of reference images shows the same photo as the one you posted as the puzzle; it has no background and no border but a caption that labels it as a 1906 30hp. If it was just a statement I'd found somewhere on the web, I'd be very suspicious of it. But it appears to be an image taken from a published work, such as a book or periodical. I can attach it here if you'd like.
RtR