Section 12. No retail instalment contract shall be signed by any party thereto when it contains blank spaces to be filled in after it has been signed, except that, if delivery of the motor vehicle is not made at the time of the execution of the contract, the identifying numbers or marks of the motor vehicle or similar information, and the due date of the first instalment, may be inserted in the contract after its execution. The buyer’s written acknowledgment, conforming to the requirements of section thirteen, of delivery of a copy of a contract shall be conclusive proof of such delivery and of compliance with this section in any action or proceeding by or against an assignee of the contract without knowledge to the contrary when he purchases the contract.