Onyxman has it. Very early combustion engines didn't have spark plugs as we know them today. These plugs would be inserted into the engine block opening and the spark would result when the piston itself got close enough to the core of wires to arc across. Usually, it was just a hole, and a metal forked clip held the plug down, hence the term "plug". They were generally connected to a crude magneto usually driven by gear or friction drive directly from the crank.
Typically, these plugs didn't last long, which is why you'll find them all over the place and in large quantities. As the compressions in engines rose, neither the "plug" method or the materials used could withstand the pressures, so the spark plug as we know it today, a threaded metal rod encased in ceramic, came along. We have early Curtiss examples dating back to 1909.
I'd place the time frame sometime between 1905 and 1910. We have an example of an early Boland engine dated 1908 that has missing plugs. I actually fabricated a pair that look very similar to this, from turned acrylic rod, to fit the engine. The openings are not threaded.
Hope that helps,
Jeff
(Former Director of the NJ Aviation Hall of Fame and Museum)