If the insurer has not transacted ocean marine insurance in this State in each of the three calendar years immediately preceding the year in which a tax return is required to be filed, its tax shall be computed on the basis of its premiums and underwriting profits in the last completed calendar year. As soon as the insurer comes within the terms of Section 12104, the taxes computed under the provisions of this section for the two preceding calendar years shall be adjusted to equal the amount of the first tax computed under Section 12104. The adjustment shall be shown as an addition to, or a credit against, the tax computed upon business done in the third calendar year or

as soon thereafter as possible.