Your dating tube
The theory that they pertain to individual batches of tubes or "production runs" seems to be the most plausible, and is the purpose I have accepted as to their meaning.
In the code used after 1955, each month is represented by a single letter in the sequence shown on the next chart.
I am constantly amazed at the amount of information you can find on the internet – including, and especially on You Tube! You name it and you can probably find it on You Tube.The year and month started to be used at this point after 1956, so the change code is not really necessary in newer tubes to pinpoint a manufacture date.The purpose of these codes, indeed all of the change code, is unclear from original Dutch Philips documentation.Amperex, Philips, Valvo and other Philips owned factories coded their tubes in a similar fashion, but their codes were not upside down like Mullard sometimes is. The sideways H is the factory mark of Siemens-Halske in Munich, Germany. Siemens never used a week of the month in their codes.Here is an example of a Siemens-Halske factory code: Note the code is broken down into 3 lines of 2 characters each. Siemens later used metal tags inside the tube on the getter support in which the code was stamped.