As near as I can figure out … Multiple internal brass barrels, with the outer-most being locked in position and having a spiral slot. The next inner barrel (which holds the pencil tip) slides freely from the barrels but has a pin that fits in the slot, and somehow, as the barrels are pulled apart it slides forward along the spiral groove until fully extended. I have one that the solder connected the barrels has come loose; I'll try and find it and take some pics to send you.
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