For the purposes of the retirement system of which a county superintendent of schools is a member, all contributions by or on account of the county superintendent and all benefits to him shall be based, insofar as, and to the extent that, such contributions and benefits are based upon the compensation received by him as county superintendent of schools, upon the whole of his annual salary as county superintendent of schools regardless of the fund or funds from which paid, subject only to the maximum amount of salary on which contributions can be paid the system. If such contributions are required to be paid to the system from any fund from which any part of the annual salary of the county superintendent of schools is paid, then contributions shall be paid from each of the funds from which his annual salary is actually paid in the same proportion of the total contribution as that part of his annual salary paid from such fund is of the whole of his annual salary. Contributions which would have been made by any county superintendent of schools, or by an employer on account of said superintendent, if this section had been in effect on and after September 19, 1947, shall be made in accordance with the provisions of this section.