Bicycle chain tensioner on a track bike, probably designed in the late 1890s, using what looks like a 1/4x20 square-headed bolt and jam nut.
The chainstay descends at about a 15-degree angle toward the bottom bracket, which means the dropout was angled downward and rearward at about 30 degrees to the horizontal, solely to accommodate the tensioner protruding from between the chainstay and seatstay.
Not as cleanly executed as the tensioner on that Cippolini carbon track frame posted two weeks ago, but no less effective. And that's the only true metric—Does it actually work? This does, and without the benefit of 100+ years of technical progress that's taken place between this bike and the Cippolini machine.