Service tube 2X63 from manufacturer Tungsram was specifically designed for identifiing a possible heater failure in serial connected heater system. There is no other function of this tube. Construction is very simple. There are two heating wires in series: 2 x 6,3V (300 mA), no cathode or anode. The metal cylinder visible in the pictures (together with the two micas) is only holding it all together. New old stock tubes which may be interesting for your collection of uncommon tubes.