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This is the small pin that holds the earpiece to the semicircular bracket, that can get lost and make your Stax earphones useless.
It is manufactured in two parts: the thin pin and the flanged cylinder that fits into the earpice. The pin is an interference fit into the cylinder, and the cylinder is an interference fit into the earpiece. First trim the hole in the flange if necessary, then gently press the pin into it with a vice or a pair of multigrips. Make sure to press it in from the flanged end, not the end that fits into the earpiece. Press it until it is flush with the other end. Then fit the cylinder into the earpiece, trim the outside edge of the pin if necessary, and fit the earpiece holder over the pin.
This piece is available in black plastic for cosmetic purposes, but the steel option is stronger, and the part is barely visible when fitted, so please consider this option. The original is some kind of fairly hard plastic.