Details
New old stock 7 pin miniature triode. Various brands. EC92 is the European designation. Essentially half a 12AT7. The 6664 is selected for mobile service (steady output at 6.3 +/- 1.3 filament volts).
NSN/FSN = 5960002620190, 5960006155583
HP/Agilent Number = 1921-0029
Tektronix Number = 154-0010-00