BD4 (Novis, Wien) - vacuum triode - rare. Measured parameters: Uf = 4V, Ua = 20V and Us1 = 0V, Ia = 5.8mA.  Measured on ELPO P-508 tube tester. Enigmatic triode with very little data available (see, e.g., https://www.radiomuseum.org/tubes/tube_bd4.html )