§ 2. The tours of duty of sergeants, roundsmen, patrolmen, (not
including detective sergeants) on duty in the open air, on the streets
or other public places of a city of the first class or a city of the
second class shall be changed at least once in each calendar month.
Provided, however, that in cities of the first and second class which on
December thirty-first, nineteen hundred and ten, had in force and
operation a three platoon system, the tour of duty of such platoons and
each member thereof, and the manner of changing such tours of duty may,
at the election of the local authorities, having the management, control
or direction of the police force, be and continue as the same were in
force and in operation on said December thirty-first, nineteen hundred
and ten.