Understanding Noise Figure of LNA (HP 461A amplifier)
Noise Figure requires a given Input Noise Density, often 50 ohms.
You may be more interested in the Noise Voltage, so you can use 300 ohm antennas, or 600 ohm audio, or magnetic beacon sensors that are just inductors.
So let us compute theNoise Density (nanoVolts/square_root_Hertz). And the equivalent Rnoise. I use Rnoise as an easy design concept, to combine with other discrete resistors in a circuit and predict the total noise.
The datasheet gives 40 microVolts INPUT, over 150MHz bandwidth. That is, this is input_referred_noise (RTI).
Now scale that by sqrt(150,000,000). For easy math, use sqrt(100,000,000) which is 10,000.
So the internal broadband noise (per root_Hertz) is

40 microVolts / 10,000 = 4 nanoVolts

Knowing 62 ohms produces 1 nanoVolt, and this math predicts 4nV, the
equivalent internal Rnoise of that first transistor is

62 ohms / (4nv/1nv)^2 = 62 * (4 * 4) = 1,000 ohm Rnoise.

Lots of transistors can achieve that noise floor.
I recall Hewlett Packard had long had access to ultra_small resolution photo-lithography machines, maybe even electron-beam machines.
I would not be surprised if HP produced special electronic systems for the US government. For national defense there really is no price limit on monitoring other nations' radio/radar emissions. Even back in the 1960s.
These systems simply are built, to avoid any surprises, at whatever cost.