This is from memory from 40 years ago, some details escape me.
To achieve an ultra high vacuum level, the tube was attached to a mechanical pump in series with diffusion pump (google the term) while simultaneously heating it in a convection oven at 250 Celsius for several hours. This roughly de-gasses the tube.
Then utilizing a RF induction heater, the gun’s metal components would be heated cherry red, with the pumps still attached evacuating any
gassy emissions.
Then the heater would be subjected to 2 or 3 times it’s rated voltage, it would glow incandescent white for a few moments. Again the pumps would evacuate any additional gas.
Only then the stem would be sealed, but the process was not yet complete. With another induction heater, the barium getters would now sputtered, producing a marvelous purple glow while depositing the silvery metal inside the tube. The barium would capture any leftover molecules.
The final pressure before the stem was sealed was measured in tenths of mili torrs but can’t recall exactly the value. A milli torr is already very small: 1/760,000 atmospheres.
I never measured the final vacuum after the getter had been sputtered, but was told it would be reduced 10 to 100 times lower. A high vacuum indeed!
By then, the Japanese manufacturing had already upgraded to turbomolecular pumps, and was anecdotally told that they had 20X lower vacuum. One of the several reasons that those tubes lasted so long.