For what it's worth, I think it was definitely for the control of the flow of something, possibly air? to what though, Lamp, organ, stationary engine??
It appears as though it has been knurled on the outer circumference of the knob end, to be turned/twisted with the fingers, which of course would rotate the other end to either open or shut an airway, or any size aperture in between. That little dog-leg on the spring on the other side would allow you to rotate the whole thing without it coming out of whatever the whole unit slides into.
Of course, depress that spring and remove and it would allow you to remove the whole unit giving full air flow and/or allowing you to clean the thing. That little bobble on the end would help you to pull it out while you depress the tab. I don't think it would be of any use for anything other than airflow though- Wine, Fuel, or water would pee out I'd think. As for what it is though FIIK :lol: