The pilot in command reported that during the loading of the seventh and final passenger, the airplane tipped onto its tail and impacted the ramp surface. He further reported that the tail stand was not installed and neither he nor the co-pilot noticed the omission before passenger loading began. According to the National Transportation Safety Board Accident/ Incident report form 6120.1 submitted by the operator; after a review of pictures sent to company maintenance, no "major damage" was visually identified, the airplane was released for service, and the flight crew completed the revenue flight. After the airplane reached a company domicile, the airplane was "grounded" after a further visual inspection revealed substantial damage to the aft bulkhead. The pilot did not report any mechanical malfunctions or failures with the airplane that would have precluded normal operation.