This Christmas tree shaped porcelain insulator was identified by collector Dan Howard as an antenna tuning coil insulator ( http://www.nia.org/notins/nons6.htm ), but there have always been lingering questions about whether they were used on ships or submarines as some sort of insulated cable tray. When I was a newbie dealer I remember seeing a couple of these insulators on another dealer's table. He said he acquired them from a woman who said they belonged to her dad who worked on Mare Island, a large naval base and shipyard. I obtained the one in the photo plus another from an antique dealer who got the pair from a woman who said the insulators came off a ship.

To my surprise I found an ink mark on the firing rest of each insulator of an anchor with the letters "U" and "S". Is it an insulator manufacturer's mark? Is it a naval mark? Can't find a similar mark on the internet. Any Naval or Coast Guard veterans have any idea?