On or before June 1, in addition to the notice of preliminary valuation published pursuant to section 77-1301, the county assessor shall notify the owner of record as of May 20 of every item of real property which has been assessed at a value different than in the previous year. Such notice shall be given by first-class mail addressed to such owner's last-known address. It shall identify the item of real property and state the old and new valuation, the date convening of the county board of equalization, and the dates for filing a protest.