DURING PREFLIGHT WALK AROUND OF A B737-300 PILOT NOTICES A DENT WITH A SHARP INDENTATION NEAR THE TAIL JACKING HARD-POINT; NEAR THE AFT PRESSURE BULKHEAD. DENT APPEARED BURNISHED; BUT NO RECORD IN ACFT HISTORY.

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ON WALKAROUND INSPECTION OF ACFT X; I SAW A DENT WITH SHARP INDENTATION NEAR THE JACKING HARD-POINT NEAR THE AFT PRESSURE BULKHEAD. IT LOOKED LIKE SOMETHING HAD BEEN POKED UP INTO THE ACFT AND THEN SCRAPED OFF. WHAT WAS ODD WAS THAT THE DENT WAS BRUSHED (BURNISHED) AS IF IT HAD BEEN WORKED; BUT I'VE NEVER SEEN A DENT LIKE THIS THAT DID NOT NEED A PATCH. THERE WAS NO 'DENT DOT' AND IT WAS NOT A PAINTED AREA. WE WENT AHEAD AND WROTE IT UP AND CALLED MAINT TO VERIFY THE DENT WAS IN THE MAINT HISTORY. TURNS OUT THE DENT WAS NOT IN HISTORY AND THE MECHS WERE A BIT CONFUSED BY THIS AS THEY; TOO; FELT THIS ITEM HAD BEEN 'WORKED.' THE DENT WAS TREATED AS NEW AND INSPECTED PER MAINT SPECS (COMPLICATED PROCESS DUE TO THE LOCATION). THIS PROCESS WAS NOT FAST; BUT THE AIRFRAME WAS EVENTUALLY REJECTED BY MAINT (GOOD CALL) AND WE MOVED TO ACFT Y WHICH WAS ARRIVING FROM THE HANGAR TO CONTINUE FLT. OUR QUESTION HERE IS; HOW COULD THIS ACFT X HAVE FLOWN SO LONG WITHOUT THIS DENT BEING QUESTIONED? IT WAS CLRLY QUESTIONABLE TO ALL OF US (INCLUDING MECHS) THAT LOOKED AT IT. ALSO; HOW AND WHERE WAS IT DAMAGED? IN ACFT Y; WHICH CAME TO US FROM THE HANGAR; THE R IRU FAULTED WITHIN 5 MINS OF STARTUP WITH NO ALIGN LIGHT. BOOK INDICATED NO TROUBLESHOOTING FOR THIS CONDITION; SO WE LEFT THE IRU ALONE AND CALLED MAINT AGAIN. TURNS OUT; THE R IRU HAD JUST BEEN CHANGED. ROBBED IRU FROM ACFT X (THE DENT BIRD). WHILE MAINT INSTALLING IRU; THE R PACK CONTROLLER FOR R PACK FAILED (PACK NOT IN USE!). ROBBED PACK CONTROLLER FROM ACFT X (OR RE-RACKED THE EXISTING -- I WAS NEVER CLR ON WHICH IT WAS); BUT ALL TESTED AND SIGNED OFF OK. THIS WHOLE INCIDENT (ACROSS IN BOTH ACFT) TOOK NEARLY 4 HRS. LET ME BE CLEAR HERE: I THOUGHT MAINT DID A GREAT JOB TODAY. THEY FIXED OR PROPERLY EVALUATED EVERYTHING THAT POPPED UP; AND GIVEN THEIR WORKLOAD; IT WAS DONE FAST ENOUGH. OUR QUESTION ON THIS SECOND ACFT Y IS; IF MAINT KNEW THE IRU WOULD FAIL OR WAS LIKELY TO FAIL; WHY DID WE NOT GET A 'HEADS UP' ON THIS FACT? WE'D HAVE BEEN FASTER TO SPIN IT UP! THIS FAILURE MAY BE COINCIDENCE; BUT WHEN I CALLED IT IN; IT WAS VERY CLR THAT AN IRU FAULT WAS ALL BUT EXPECTED. I WOULD HAVE BEEN FINE WITH THE PLAN OF SWAPPING IRU'S AT THE GATE; BUT WAITING FOR US TO RPT THE FAULT COST US A LOT OF TIME.