Except as provided in subsection (b) of this section, any motor vehicle master key, any pattern, impression, or mold from which a motor vehicle master key may be made, or any advertisement for the sale of any such key, pattern, impression, or mold, is nonmailable matter and shall not be carried or delivered by mail.

The Postal Service is authorized to make such exemptions from the provisions of subsection (a) of this section as it deems necessary.

For the purposes of this section, “motor vehicle master key” means any key (other than the key furnished by the manufacturer with the motor vehicle, or the key furnished with a replacement lock, or any exact duplicate of such keys) designed to operate 2 or more motor vehicle ignition, door, or trunk locks of different combinations.