The airline transport pilot was ferrying the airplane under Title 14, CFR Part 91, to an airport to be painted. During cruise flight, the upper portion of the aft cargo door separated from the airplane, striking the horizontal stabilator. The pilot said he believed the door was latched securely prior to the flight. The flight continued to the destination airport and landed uneventfully. An examination of the fuselage-mounted upper door latch by an FAA inspector showed that the upper portion of the latch striker plate was missing, and the door locking mechanism was not properly aligned with the latch. The FAA inspector noted that it appeared that the missing portion of the latching plate had been missing long before the accident, and should have been found on routine maintenance inspections. The upper portion of the door was not recovered. The airplane sustained substantial damage to the stabilator.