Details
New old stock 7 pin miniature duodiode. Various brands. EAA91 is the European designation. Type 6663 is selected for mobile service (steady output with a heater supply of 6.3V +/- 1.3V).
NSN/FSN = 5960001269664, 5960001883544, 5960005561308, 5960008782506 (6663)
HP/Agilent Number = 1930-0013, 1930-0024, 5080-0018
Tektronix Number = 154-0016-00