According to the invention, a method for authenticating download of a number of digital content files ordered from a web site is disclosed. The board has an inner surface and an outer surface. The supporting members have extensions projecting beyond the swivel joints thereof. The display frame may lie under the inactive area. With the above system thereof, the imaging inspection of the moving object with rapid speed can be undertaken safely and reliably. The user continues to rotate the retention ring until retention ring is placed into a locked position.