PPC did not specifically request the gold marks to be applied on the sides of the lugs, but rather, reluctantly did so for customs compliance in Geneva.
It very much depended on how stringent the officer in charge at the time, and it is not uncommon that he or she would request the gold marks to be applied to highly visible parts of the case, e.g. the sides of the lugs. Apparently they would even go to the extent to request PPC or the case maker to manufacture special tools to allow stampings in highly unusual angles.
This feature is usually found on pieces made in the early 1990s. The case was either made by JHP or Ateliers Reunis. Subsequently all gold marks were applied to underneath the lugs for aesthetic reasons.
Hope that helps.