RPTS OF INCORRECT FMC PERFORMANCE SOFTWARE INSTALLATION ON B737 ACFT.

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INCORRECT SOFTWARE LOADED IN FMC. AT MY AIRLINE WE FLY 4 DIFFERENT MODELS OF THE B737. ON THIS DAY WE WERE IN A B737-500. THE DISPLAY ON THE FMC WAS DIFFERENT THAN OTHER MODEL '500'S.' AT FIRST IT APPEARED THAT THE DIFFERENCE WAS JUST A SOFTWARE UPDATE CHANGE. IN FACT THE SOFTWARE LOADED WAS FOR A B737 MODEL '700.' MAKING A FULL PWR TKOF AND USING V SPDS AND APCH VREF SPDS FROM A SEPARATE COMPUTER ELIMINATED ANY ERRORS. HOWEVER; WITH THE WRONG SOFTWARE LOADED; IT WOULD BE POSSIBLE TO USE INCORRECT PWR SETTINGS OR REF AIRSPDS. 2 PERSONAL OBSERVATIONS ON THIS OCCURRENCE. FIRST; I FIND IT HARD TO BELIEVE THERE IS NOT A FAILSAFE DESIGN IN PLACE TO PREVENT LOADING THE WRONG MODEL SOFTWARE. SECOND; BECAUSE I HAD FLOWN THE MODEL '700' THE PREVIOUS DAY; SEEING THE WRONG DISPLAY IN A '500' DID NOT IMMEDIATELY RAISE A WARNING FLAG IN MY MIND. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS RPTR STATES ON CALLBACK THAT HE WAS SURPRISED TO LEARN THAT PERFORMANCE DATA MEANT FOR DIFFERENT VERSION ACFT COULD BE LOADED INTO THE WRONG FMC. RPTR FURTHER STATES THAT THERE IS NO OBVIOUS INDICATION IN THE COCKPIT OR FMC CDU THAT DRAWS ATTN TO THE VERSION OF SOFTWARE LOADED ON THE FMC. HE ALSO RPTS THAT HE BELIEVES THIS SIT HAS BEEN ADDRESSED BY HIS COMPANY AND THAT HE HAS NOT HEARD ANY RECENT RPTS OF THIS SAME PROB.