The applicable part of the county assessment rolls as made by the county assessor and equalized or corrected by the board of supervisors or the State Board of Equalization shall be the basis for levying the district’s assessments; provided, that in any case where the board of directors of a district has, prior to November 8, 1967, adopted a formal resolution or entered into a written agreement with an owner of land within the district providing a different basis for levying the district’s assessments against the land of such owner, such resolution or agreement shall be the basis for levying the district’s assessments.