An employer shall, when requested by an employee upon the termination of his employment, supply such employee with a certificate of service showing full names of the employer and employee, the nature of the employment, the dates of commencement and termination of the contract and the rate of remuneration at the date of such termination:

Provided that where in the Main Agreement the wage of any employee is determined by length of service, it shall be incumbent on the employee to produce a certificate of service to his new employer on change of employment in order to become entitled to such remuneration prescribed for length of service.