Should any lots or lands be divided after a special assessment thereon has been confirmed and divided into installments, the assessor shall apportion the uncollected amounts upon the several lots and lands so divided, and shall enter the several amounts as amendments upon the special assessment roll. The City Treasurer shall, within ten days after the apportionment, send notice of the action to the persons concerned at their last known address by first class mail. The apportionment shall be final and conclusive on all parties unless protest, in writing, is received by the City Treasurer within 20 days of the mailing of the notice.