Very rare tube. It is a very early release of the GEC L63 gray glass ST shape. The L63 is the European designation of the 6J5/G. The 6C5/G preceded the 6J5Gm and the later tube is electrically equivalent so the two are interchangeable. Either these tubes were mislabeled as 6C5G or Cossor intentionally labeled them as 6C5G because it was acceptable for tube manufacturers to label a newly released tube with a more commonly recognizable and equivalent marking. Nevertheless, these are GEC L63 tubes as the internal construction exactly matches other L63 gray glass tubes, and the tube tests as an L63/6J5 on the tube tester. Finally (and most importantly) I have verified that the sound of this tube exactly matches the sound signature of the later tubes with the L63 markings.

Ok, all of that out of the way, here's your chance to own a pair of ultra rare L63 tubes. They are widely regarded as the best of the 6J5 family. I like them so much that I kept 8 for myself. I had six available for sale and now down to these last two. They have been tested and matched with my calibrated Jackson 648-S tester.