Details
New old stock 9 pin miniature twin triode. The 5751 was designed for the Air Force to be exactly a 12AX7 with 70% of the gain of a standard 12AX7.
NSN/FSN = 5960000199171, 5960000271720, 5960000685801, 5960000714601, 5960000733442, 5960000824139, 5960001935145, 5960002256753, 5960002962225, 5960005561927, 5960005819431, 5960006472998, 5960006696862, 5960007046372, 5960007046436, 5960007059094, 5960008000518, 5960008278779, 5960008466578
HP/Agilent Number = 1932-0004, 5080-7334, 5080-7335
Tektronix Number = 154-0094-00