“Local police officer” also includes any officer or employee of a juvenile bureau of a contracting agency whose principal duties consist of active law enforcement service, except persons whose principal duties are clerical or otherwise clearly do not fall within the scope of active law enforcement, even though the person is subject to occasional call, or is occasionally called upon, to perform duties within the scope of active law enforcement.

The provisions of this section do not apply to the employees of any contracting

agency having a contract with the board made prior to September 22, 1951, until the agency elects to subject itself and its employees to the provisions of this section by amendment to its contract with the board pursuant to Section 20474; except that an election is required only among the employees to whom Section 20427 applies.