Cal. Rev. & Tax. Code § 276.5

If a person claiming the exemption of an aircraft of historical significance under Section 220.5 fails to file the affidavit required by that section by 5 p.m. on February 15 of the fiscal year for which the exemption is claimed, but files that affidavit on or before the following August 1, the assessment shall be reduced by an amount equal to 80 percent of the reduction that would have been allowed had the affidavit been timely filed.