This is very rare find, once in a lifetime!
Factory matched pair, very tight readings.
1958 date codes, black plates, triple mica construction.
One base is slightly lighter color brown, which is common in this era.
Plate current on the Maximatcher @ 400v \ -48 = 35.5mA \ 35.3mA
Hickok 600A Readings are very strong, proper test procedures for 6550 were done with care. 7000 is nominal on the Hickok
11,500 \ 11.250 Steady @ 6.3 Filament Voltage
11,500 \ 11,250 Steady @ 5v Drop