A PA18-150 WAS DISCOVERED TO HAVE MODIFICATIONS PERFORMED ON THE AIRPLANE WITH NO 337 FORM PAPERWORK TRAIL.

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THE PURPOSE OF THIS RPT IS TWOFOLD: MOST IMPORTANTLY IS THAT I MISSED DOCUMENTING PREVIOUS MODIFICATIONS TO AN ACFT BY RELYING ON THE OPERATOR'S RECORDKEEPING. SECONDLY; IS THAT THE LCL FAA'S OVERZEALOUS AND ANTAGONISTIC ATTITUDE PREVENTS ME FROM BEING ABLE TO OPENLY DISCUSS THIS MATTER WITH THEM WITHOUT HAVING AT LEAST SOME PROTECTION FROM RETRIBUTION. THE NEW DIRECTOR OF MAINT INSTRUCTIONS INCLUDED REVIEWING THE ACFT'S MODIFICATION STATUS AND MAKING A LIST OF SUSPECTED MODIFICATIONS AND REWEIGHING THE ACFT TO UPDATE THE WT AND BAL AND EQUIP LIST. DURING THIS REVIEW I FOUND MANY PREVIOUS MODIFICATIONS; THAT UPON CHKING WITH THE MAINT BASE IN ZZZ1 WERE DISCOVERED TO NOT HAVE ANY PAPERWORK TRAIL. DURING PREVIOUS INSPECTIONS (WHICH I HAD SIGNED OFF) I WAS ASSURED THAT THE 337'S WERE UP-TO-DATE. ALL OF THE HISTORICAL RECORDS ARE MAINTAINED AT THE OPERATOR'S HANGAR IN ZZZ1. NOW; HOWEVER; I REALIZE THAT THE RECORDS WERE NOT COMPLETE. THE MODIFICATIONS TO THIS PARTICULAR PA18-150 ARE ALL COMMON MODIFICATIONS THAT ALMOST EVERY PA18-150 IN ZZZ1 INCORPORATES. I WAS AS SURPRISED AS THE ZZZ1 MAINT STAFF AT THE LACK OF PAPERWORK. TOGETHER WE ARE REVIEWING ALL OF THE RECORDS FOR THIS ACFT AND UPDATING THEM.