Before revision: A brief summary of just the more common letters is: ECC81 / \ \\__ last digit(s) give serial number / \ \__ first digit(s) indicate base (3=octal , 8 or 18 or 80=Noval (B9A), 9=Miniature 7-pin (B7G).
After revision: A brief summary of just the more common letters is: ECC81 / \ \\__ last digit(s) = serial number / \ \__ first digit(s) = base (3=octal / 8 , 18 , 80=Noval (B9A), 9=Miniature 7-pin (B7G).