Within thirty days after the final passage of the said ordinance, the president of the city council and the mayor of the city shall appoint a commission to frame a new charter consisting of fifteen registered electors of the city, nine of whom shall be appointed by the president of the city council and six of whom shall be appointed by the mayor: Provided, however, That not more than six of the members appointed by the president of the city council of the city and not more than four of the members appointed by the mayor of the city shall be enrolled members of the same political party.
53 P.S. § 13103