This tube is Engineering Sample made by National Union. It states to be the "GI-11B". The tube looks very similar to the GI-10, but also has a additional pin which can be used as an anode! The digits are hand made like in the GI-10 and GI-21 and roughly about 13mm high. I don't have any more info about this tube, and can not validate if its a "real" sample, but since its different from the GI-10 i believe that its a real. Its unknown how many of them were made, and if they ever have left the lab! This is a super rare piece of nixie history! If you have more info about this tube, i would be amazed to hear about!