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Belay that -was nodding off or something! :D

Since I have a bolt near at hand this time I just took it down and yes, it all comes back: there is a collar that threads into the bolt head body and...

Have you not been able to remove the firing pin and spring?

Keep fiddling with it and you'll get it. Probably there are burrs on the internal interrupted threads that are catching. Once you get it apart take a close look and remove those with needle files...