The registrar of titles is hereby authorized to destroy instruments of encumbrance which have been satisfied of record or extinguished by operation of law for a period of five years together with the assignments and satisfactions thereof. When the discharge of an encumbrance is by virtue of a judicial or statutory sale, the instruments evidencing the encumbrance of the foreclosure thereof, shall not be destroyed until six months after entry of an unappealed order for issuance of a new certificate of title to the purchaser at such sale or to the purchaser’s assignee. Nothing herein contained shall relieve such registrar from maintaining the books and index records required under sections 508.34 and 508.37.