The StarWalker Rubber has no serial number engraved on the cap. That is normal for this model and no indication of a fake. On other StarWalker writing instruments the number is applied to the side of the clip, and so finely that it is barely visible to the naked eye; without a loupe and raking light you will often search in vain. Anyone who cannot find the number should therefore look there first before doubting their pen's authenticity.
In general the serial number is not a consistent test of authenticity at Montblanc. There have been, and still are, models and whole editions delivered without any such marking, and conversely fakes now carry invented numbers of their own. More telling are the workmanship, the weight, the feel of the materials, the execution of star and clip, and, on fountain pens, the nib and the feed. In any case, a number that is present can only be decoded by the manufacturer itself, since the code is not publicly documented.