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Back around 1967 tubes diagonal measure switched from bulb size to viewable phosphor size and tubes went from having 2-4 letters in the middle to having 5 letters in the middle of the number. The first numbers are diagonal measure and the last are phosphor type 22 being the common value for color tubes and 4 being the common letter for monochrome(there were other numbers, but they were usually for special purpose gear).
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