Details
New old stock 7 pin miniature sharp cutoff pentode. Various brands. E90F is the European designation. Type 6661 is selected for mobile service (steady output at 6.3V +/- 1.3V on the filament).
NSN/FSN = 5960001883602, 5960005534523, 5960005726214, 5960006858998 (6661), 5960008095038, 5960008543362 (6661)
HP/Agilent Number = 1923-0027, 5080-7339
Tektronix Number = 154-0026-00, 154-0285-00