Any provision of a retail installment contract as originally written or subsequently amended which purports to provide for the inclusion of title to or a lien upon any goods subsequently or previously sold under a retail installment contract not paid in full, other than that originally sold as the subject of such specific installment sale or other than substitution in whole or in part therefor, as security for payment of the time sale price or any part thereof shall be void; but the other provisions shall not be affected thereby.