RPTS OF INCORRECT FMC PERFORMANCE SOFTWARE INSTALLATION ON B737 ACFT.

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DURING PREFLT; THE FO OBSERVED AN UNEXPLAINABLE LINE ITEM ON THE TKOF REF PAGE OF THE ACFT FMC/CDU. IT NOW APPEARS THE ENTRY MAY HAVE BEEN THE RESULT OF AN IMPROPER DATABASE HAVING BEEN LOADED IN THE FMC. THE PERFORMANCE DATA WAS CHKED AND VERIFIED AGAINST THE ACFT'S PERFORMANCE COMPUTER. NO DISCREPANCIES WERE NOTED. THE ACFT WAS THEN FLOWN 2 LEGS; STL-TUL; TUL-PHX WITH NO DISCREPANCIES NOTED. I HAVE SUBSEQUENTLY BEEN ADVISED THAT THE CDU HAD PREVIOUSLY BEEN INSTALLED IN A B737-700 BEFORE BEING INSTALLED IN ACFT AND HAD NOT BEEN PROPERLY REPROGRAMMED FOR USE IN THE 500 SERIES ACFT. ALTHOUGH THE ACFT PERFORMANCE COMPUTER RECOGNIZED PROPER 500 DATA WAS INSTALLED; IT DID NOT (WAS NOT ABLE) TO UPDATE ITSELF TO ELIMINATE THE UNEXPLAINABLE LINE ON THE TKOF REF PAGE. TECHNICALLY I USED AND FLEW THE ACFT USING IMPROPER COMPUTER INFO (THE DATABASE WAS A COMBINATION OF 500 AND 700 DATA); BUT BECAUSE THE FMC WAS IN FACT 500 SPECIFIC AND WAS VERIFIED BY THE PERFORMANCE COMPUTER; THE ACFT WAS OPERATED SAFELY.