Rare - Artist Proof - Stamped AP - Less than 25 known to exist. According to PinPics: Artist Proof. Artist Proof pins (or AP pins) are created during a manufacturing run to verify quality. AP pins have an AP stamped on their back.
Generally 20 - 24 AP pins are made of each pin per run. Artist Proof pins can differ in style or be exactly the same as regular production/release, depending on what ultimate design is chosen to move forward with in mass production. Please feel free to ask any questions you may have.