According to Pinpics: An Artist Proof pin, or AP pin, is a pin that is used to verify details of a pin before it is released to the public. Generally, 24 AP pins will be created for each pin produced. If a pin is of a lower edition size, the number of AP pins made may be lower. On rare occasions if a pin is cancelled at the last minute due to an oversight, the AP run will have been made already and will still be put into circulation. These pins will have an "AP" stamped on the back of the pin somewhere.
Because Disney uses different manufacturers and different styles of backstamp, the location and size of the "AP" stamp will vary.