Here is a picture of a few porcelain neon sign insulators. I guess these would fall into the tube/wall-tube/panel-thru type insulator category. Some people mistake them for lightning rod insulators. They would fit through a hole in the sign panel and were held in place by the clip snapping into place on the inside of the sign. The glass neon tube would then pass through the center hole of the insulator. I removed a total of eight of the pink ones from a neon sign myself about 25 years ago. The black glazed one was secured from eBay a few years ago. I also have a clear glass pyrex one. The porcelain ones apparently came in a numer of different colors to match the main color of the neon sign. I have seen cobalt blue, white and different shades of brown glazed ones.