JustaFew,
But either, more are needed, or someone could reinvent the transponder to use all 10 digits. By missing out 8 and 9, several thousand codes are NOT usable ( 5904 to be exact!).
For the benefit of the non-anoraks, the use of all 10 digits would have far reaching development implications as the whole code system was (is?) based on the octal base, ie 0-7. With a bit of luck someone will be looking to the future!
Sorry for the nerdy comment
