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It's that shiny spot that made me think it might be soldered. The melting point of BT alloy, which is a eutectic, is 780C. Other alloys are higher.
There must be a clue in the fact that the end nearest the base is darkest and gradually get lighter as you move away from it. I wouldn't expect that temperature distribution from induction heating or activating the cathode.
Maybe another possibility. I forget what you call the glass disk feed-through portion. It's basically a disk until after they mount the gun. Then it's attached to a tube that will later form the neck. Or perhaps done in one step.
When they do that joint that end of the gun will get hot. It could oxidize during that step, depending on how they do it. If they join it to the end of a tube then it would see some flame and get hot. If they shrink a tube down around it and then remove the excess, then it would stay fairly cool.
Does that make any sense?
John
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