With the model code:
The model code is the production code for Royal Enfield vehicles and includes all informations (model, year, finish) specifics to your vehicle. This is the most efficient way to be sure to find your model and do not make order mistakes !
With the chassis number or VIN:
If you want to identify your vehicle more accurately, you need your vehicle's chassis number, which is different on each vehicle and will help you to find informations the model code does not contain, very useful if you do not know the colour code for example.
Be careful, the date on the registration document is the date of 1st registration, which could be different with the model year.