I am completely indebted to and grateful to Letizia Jacopini for her pictures of this very interesting pen. It is a twist filler with a threaded knob, similar to the AA Waterman. But the knob is attached to an external sleeve with a zig zag cut which fits into a matching zig zag locking ring.
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![](https://vintagependoctor.com/wp-content/uploads/2022/06/Nettuno-twist-fill-parts.jpg)