The Commission shall maintain a list of persons eligible for appointment in each class or position in the classified service. Each list shall remain in force for one year from the date of its promulgation unless a new examination is held sooner, or unless the time of such expiration is extended by the Commission. Such extension, if made, shall be entered on the minutes of the Commission with the reasons therefor. No such extension shall be longer than six months and no more than two such extensions shall be made on the same list.