I have seen it on aluminum tubes for bicycle trailers, outdoor gear, collapsible shovels, baby jumpersthe two tubes are fitted so one slides over the other and this piece of hardware locks them together.
Various names: Spring Pins, Spring Plungers,Ball Nose depends on where and how they are usedMost of these are used to lightly hold something in position like a cabinet doorBut I think this is what you are looking forThe McMaster Carr catalog calls them by the technical name Quick-Release Button Connectors for Telescoping Tubing Part Number 92988A730
