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The purpose of these codes, indeed all of the change code, is unclear from original Dutch Philips documentation.
The clip above will give you a demo, but it's simple to execute if you're keen to try it out.
Remember, this code element follows the tube type code and the change code, which are always the first two codes in the code sequence. This code is really only necessary if the factory label becomes rubbed off of the tube, so you can tell what tube you have. Most audiophiles are concerned with date codes and, to a lesser extent, the factory code.
This is for reasons of finding tubes that could be put together in matched pairs.
For the most part, tubes made after 1956 are what is commonly found on the vintage market today.
This will reduce the number of code choices you have, and the tube type codes of the tubes you use the most will become committed to memory, as they never change.Create your free profile now and start meeting colorful singles with whom you can be yourself.