The simplest vacuum tubes resemble incandescent light bulbs in that they have a filament sealed in a glass envelope which has been evacuated of all air.When hot, the filament releases electrons into the vacuum: a process called thermionic emission.Vacuum tubes were simply a commodity that everyone needed like batteries or light bulbs are today.For the most part, back then no one cared when a tube was made or who made it, they just wanted a good product that would work and bring their radio, amplifier or TV back to life.The following description is applicable to Mullard and Philips vacuum tubes, it may also apply to other manufacturers but only by coincidence.Generally (and I can't express that enough) a Mullard or Philips vacuum tube will be marked near the base with a 2 row code, the first row is the manufacturers type code and the second row is a country/date code.The electrodes are attached to leads which pass through the envelope via an air tight seal.
These are codes often seen stamped on components found in US made vintage tube equipment.
Which is part of the reason it can be so hard to figure out today.
There were a few different reasons a given company would re-brand or write the wrong country of origin on a tube.
When trying to identify a tube it makes the most sense to first determine where it was made. Depending on where it was made, you there are different clues to look for.
Once you know where it was made, that narrows down the possible manufacturers by a lot.