I have seen many, many models with different rivets over the years. Sometimes two identical models can have quite different rivets, presumably down to the alignment, or misalignment of the tools used, and also maybe different production lines that might be running at the same time to meet demand. E.G. two 75a identical Ferrari Berlinettas I own have one standard rivet and one which has been stamped to the side, missing half the rivet but still holding the base to the model. Many other such examples are readily found.
The 56a in question would need to be held in hand to make an absolute decision either way, but from what can be seen in the photo, and that includes the same colour on the underside of the wheel arches, I would be more inclined to say it is genuine than not, just my opinion of course
