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.Obviously goes as 1=first week, 2= second week, etc.We take discretion and privacy extremely seriously and our members are asked to sign a Privacy Agreement when subscribing to our services.We can arrange an informal, no obligation first interview with one of our consultants either at our offices in The Hague or by telephone call.
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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.
Remember, the last digit of the year began to be used with this code, and this will help identify post 1955 tubes.
This is the actual week of manufacture within the listed month.