These are insulators, but have previously been thought to have been used on submarines. An article "Designs and Efficiencies of Large Air Core Inductances" from the Dec 1925 "Proceedings of the Institute of Radio Engineers" shows these to be insulating supports of large radio tuning inductors. The examples shown in the article all appear to be land based, but one could assume they could be used on large ships as well. Thanks to Dan Howard for this article.
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