The suffix letters specify the type of envelope or base. The code is G large Glass envelope. Consists of digits, Letters, digits. Do not confuse with a US code. Mazda codes tend not to double up letters - L is used for multiple triodes also.
Final digits distinguish between otherwise identical codes.
British Royal Air Force Code consists of letters followed by digits. Again, letters followed by digits. Another letters followed by digits code. There is no way to decode these by simple inspection of their number. ZA and 10E are also used as prefixes to catalogue numbers.