As the acft was climbing thru 14,800' for 19,000', the plt reported a rapid decompression. Two minutes later, he reported that he has lost the tail. Acft impacted in an inverted flat spin and the tail and a section of empennage were found about 3/4 mile away from main wreckage. Investigation revealed that a skin seam had split open along a 54' span in an area where 2 pieces of 'L' shaped channel had been riveted together to form a stringer. Normally the stringer would be made using a single piece of 'T' shaped channel. The non-standard stringer had been fabricated during repairs made on the acft following a gear up landing 7 yrs prior to this accident. Analysis revealed that the rivets holding the 'L' channels together and failed in fatigue due to tensile cyclic loading whenever the acft was pressurized. Acft had a history of pressurization problems following the repair that was not properly performed or inspected.