Actual pictures. To your attension a rare one edition of 6sl7(1579) tube by MELZ, the tube marked as "prototype", produced in jan 1953
Tested with L3-3 tubetester at Ua=250V, Uf(dc)=6.3V , Ug=-2.
Results: 1tr 1.95mA , 152mA/V
2tr 1.95mA , 154mA/V