I might as well throw it out there right now - but I thought of this inventor more than two days ago and I'm still not convinced... although the hint of your machine being from north of San Fran makes it hard to ignore... Lyman W. Gilmore Jr.
American steam-powered flying machine.
Check.
The design (or at least a variant of it) was patented, but not in 1904. (
One of his patent drawings, dated 1898, vaguely resembles the Challenge machine).
Check.
He was also granted a patent for a steam engine.
Check.
It was built and tested at a spot north of San Francisco, California. (Grass Valley, CA).
Check.
It's not listed anywhere within AeroFiles. (Gilmore is, but nothing he built during the early years of his aeronautical career).
Check.
The machine I'm exampling dates from 1902, not '03 or '04 AFAIK (claimed by Gilmore to be flown May 15, 1902) but he could have built something a year or two later that Paul has discovered.
Okay, after writing this I'm a little more convinced - mainly because the patent drawing I linked to (its file name is Helicopter.jpg). So that being said, your Challenge photograph represents the:
Gilmore vertical take-off steam-powered monoplane of 1903-04
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Cheers