On some 146 examples the model number actually approximately agrees with the length in millimetres; however, this is no general rule and does not apply to all models or product types. Thus, e.g., the ballpoint 164 is not 16.4 cm long. The dimensions of individual models have additionally slightly changed in the course of decades, which makes the model number as a length indication additionally unreliable. More plausible is the assumption that the numbering rather orients itself on the diameter or on the length-to-diameter ratio, without giving the exact diameter.