The M-plate is one of the identification plates found on VW busses. Short for Mehraustattung (optional extras in German), it contains production codes with information about equipment, manufacturing date, destination, specs and more.
Before model year 1977: the M-plate is located on the metal partition behind the driver's seat (passenger's seat for RHD vehicles).
Image modified from The Samba forum
Model year 1977 and after: the M-plate is located under the dashboard, on top of the metal fresh air pipe.
Use the automatic decoder to find out more about the original configuration, equipment and extras for a Type 2 bus. Simply choose the model year and fill in the M-plate fields.