deciphering GE tube date codes.........

[drh4683]

Does anyone know or have a list of how GE dated their tubes. GE did away with the year-month coding and went to a two figure alphabetical system such as:

"RY" or "KJ" etc.

And under that would be the EIA number, which was 188 (which was GE)

Ive found a few websites that mention this, but do not have any information on deciphering the letters. Supposedly the reason for this change was to prevent the customer from identifiying an "old tube" in a "new TV" for example. Curious if there is a published list somewhere?