Cal. Rev. & Tax. Code § 24963

In the case of property acquired before March 1, 1913, if the basis otherwise determined under this part, adjusted (for the period before March 1, 1913) as provided in Section 24916, is less than the fair market value of the property as of March 1, 1913, then the basis for determining gain shall be such fair market value. In determining the fair market value of stock in a corporation as of March 1, 1913, due regard shall be given to the fair market value of the assets of the corporation as of that date.