[With regard to any county which is required to operate on a countywide system of road administration as described in Section 19-2-3, this section shall read as follows:]
Where bonds have been issued by any road district, separate road district or special road district, and said bonds and interest thereon have been paid and there remains in the sinking fund or maintenance fund of such district any balance of funds collected for the purpose of paying such bonds or interest, the board of supervisors of the county in which such district or districts are located shall transfer any or all of said funds to the county road or bridge fund to be used and expended in working and maintaining the roads of the county.