Cal. Rev. & Tax. Code § 5768

Upon request of the assessor, an owner of racehorses of a type subject to the tax imposed by this part shall make available at his principal place of business, principal location or principal address in California or at any place mutually agreeable to the assessor and the owner, a true copy of business records relevant to the number and type of racehorses located in any county of the state during any taxable period and the number of days spent in each county during that period. Records referred to in this section shall be retained by the owner for a period of five years from the date any tax to which they relate becomes due.