A B727-200 FLC TAKES THE WRONG NUMBERED ACFT OUT ON THEIR SCHEDULED FLT FROM LIT; AR.

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STARTED PAIRING ON MAY/XA/99. I HAVE BEEN FLYING THE SAME PAIRING FOR THE MONTH OF MAY. THE PAIRING STARTED IN THE MORNING WITH AN STL-DTW-STL TURN. A PLANE CHANGE IN STL TO FLY TO LIT. IN LIT WE TURNED THE PLANE OVER TO ANOTHER CREW AND WENT TO THE HOTEL. IN THE MORNING; ON DEP FLT; THE SAME SERIAL NUMBER OF B727 THAT WAS DELIVERED THE NIGHT BEFORE ON ARR FLT FROM STL-LIT WAS OUR AIRPLANE IN THE MORNING. ON THE NEW MAY/XA/99 PAIRING FLT STL-LIT WAS CANCELED. WE DEADHEADED ON A LATER FLT (STL-LIT). IN THE MORNING OF MAY/XB/99 WE DEPARTED ON THE AB00 MORNING VAN FROM THE HOTEL. WHEN WE ARRIVED AT THE ARPT AT APPROX AB20-AB25; WE HURRIED AS THE CAPT TOOK THE PAPERWORK. I WALKED TO THE AIRPLANE; LOOKED AT THE TAIL NUMBER. I MENTALLY XCHKED THE TAIL NUMBER TO WHAT WE HAD FLOWN THE DAY PRIOR ON ARR FLT (STL-LIT) BUT THAT HAD BEEN CANCELED; SO IT COULD NOT BE THE SAME ACFT. IN LIT; WE HAVE 1 GATE; BUT UNKNOWN TO ME; THERE WAS ANOTHER ACFT PARKED OFF THE GATE IN THE CARGO AREA. WE HURRIED TO GET THE FLT OUT ON TIME TO STL ON DEP FLT. IN STL WE DISCOVERED WE HAD THE WRONG ACFT NUMBER. THE WRONG ACFT SERIAL NUMBER HAD BEEN PARKED AT OUR ONLY GATE IN LIT. SUPPLEMENTAL INFO FROM ACN 438375: THE VAN DRIVER WAS LATE AND COULD NOT GET THE FIRST SHUTTLE STARTED. SO WE TOOK THE OTHER BUS TO THE ARPT. BY THE TIME WE GOT TO THE ARPT WE HAD ABOUT 30 MINS (MAYBE 25 MINS) TO GET GOING. THE B727 WAS PARKED AND HAD PWR ON IT. I DID MY PREFLT; CHKED THE LOGBOOK AND IT WAS THE CORRECT ONE FOR THE ACFT. WHEN WE LANDED; MAINT ASKED IF WE HAD THE RIGHT ACFT AND THAT'S WHEN WE DISCOVERED IT WAS NOT. I GUESS THE OTHER B727 WAS PARKED IN THE DARK ON THE OTHER SIDE OF THE RAMP; AND ALL 3 -- CAPT; FO; AND ME -- DID NOT CATCH IT.