By way of derogation from article R. 25-1 of the Electoral Code, when the municipal councillors have been elected under the conditions laid down in articles L. 252 to L. 255-1 or the fourth paragraph of article L. 261 of this code and one or more deputy mayors are elected, the population figure to be used is the authenticated municipal population figure taken into account at the time of the last full renewal of the municipal council.