Any employer of 6 or more persons in this State engaged in the manufacture of any article, who shall pay any person engaged in such manufacture an unequal wage for equal work, by time or piece work, than is being paid to any other person employed in such manufacture, shall be guilty of a petty offense and shall be fined not less than $25 nor more than $100; provided, however, that nothing herein contained shall prohibit a variation in rates of pay based upon either difference in seniority, experience, training, skill or ability, or difference in duties or services performed (whether regularly or occasionally), or difference in availability for other operations, or any other reasonable classification, excepting difference in sex. Provided, further, that nothing herein contained shall prohibit such variation where the same is authorized by a contract between an employer and a recognized bargaining agent.