The seller shall not obtain the signature of the buyer to any contract when it contains blank spaces of items which are essential provisions of the transaction except as provided in RCW 63.14.060: PROVIDED, HOWEVER, That if delivery of the goods is not made at the time of the execution of the contract, the identifying numbers or marks of the goods or similar information and the due date of the first installment may be inserted by the seller in the seller's counterpart of the contract after it has been signed by the buyer.