From the 1971 model year onward, European-built Beetle saloons included a production plate, and where it remains on the car, it serves as the key link for identification of the vehicle. The finite verification of a Beetle without any original paperwork falls to two things: a Volkswagen Classic Parts Birth Certificate and double-checking the declarations on that certificate and against the evidence you may have already discovered from the car itself. The ‘production plate’ is located on the front apron of your car, painted in body colour, adjacent to the bonnet lock.
The code formatting varies slightly with era, depending on the model year of your car (that is, the August to July production year known variously as Baujahr (BJ), Build Year or Model Year rather than calendar year).