Notwithstanding the provisions of section 7-1, any town may adopt an ordinance, in the manner provided by section 7-157, requiring that a special town meeting be warned by the selectmen on application of at least fifty inhabitants qualified to vote at town meetings, such meeting to be held within twenty-one days after such application is received by the selectmen; provided nothing in this section shall be construed to affect any ordinance legally adopted prior to October 1, 1957.