Section 39L. No local body shall charge for water or sewer services on a descending unit rate basis. For the purposes of this section a local body shall be a city, town, district, commission or other political subdivision or instrumentality of the commonwealth which is not within Hampden county and which is responsible for providing by itself or through an officer, board, department or division thereof local water supply or local sewer services; except as otherwise expressly provided herein, in any case where local water supply or local sewer services within the territorial boundaries of a local body are provided in whole or in part by a political subdivision or public instrumentality of the commonwealth separate from such local body, the term ”local body” as used in this section shall mean, within the service area thereof, the political subdivision or public instrumentality.