I tried to get the YKG2S2 issued against my car in NSW because I didn't think a 4 digit number was good enough to show my car is a Cooper S. I also believe that there must be other early cars out there with the same 4 digits. But in NSW they said it has to be what is stamped into the cars bodywork.
I have just checked what VIN the following cars from QLD and VIC are registered with. Mixed results. Vic reg JRB166, SIO266, IDC111, MCS68 are numbers only. QLD reg PQR781 & COOPERS are numbers only but PIA987 & 00BMC Letters and numbers. So to answer the question of what VIN to use when going to register the car. flip a coin. It looks like QLD has used both styles. Sorry I could be of any real help.