DC8 CREW DEPARTED WITH AN IMPROPERLY DEFERRED MEL ITEM.

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PRIOR TO FLT; FOUND A LOGBOOK ENTRY DEACTIVATING THE ESCAPE SLIDE IN ACCORDANCE WITH AN ENGINEERING ORDER. ESCAPE SLIDES ARE NOT LISTED IN OUR MEL. I SOUGHT GUIDANCE FROM MECHS ON DUTY; MAINT HEADQUARTERS; DISPATCHER AND CHIEF PLT'S OFFICE. ALL STATED THE DEFERRAL WAS PROPER AND OK FOR CONTINUED FLT. LATER; I LEARNED THE ESCAPE SLIDE WAS SUPPOSED TO BE REMOVED FROM THE ACFT; WHICH IT HAD NOT; THUS; THE ACFT MAY NOT HAVE BEEN LEGAL FOR FLT. SUPPLEMENTAL INFO FROM ACN 447132: PRIOR TO FLT; DURING THE PREFLT INSPECTION; A WRITE-UP IN THE ACFT LOGBOOK SHOWED A MAINT DEFERRAL FOR THE ACFT ESCAPE SLIDE. THE CORRECTIVE ACTION CITED AN ENGINEERING ORDER AS THE AUTH FOR THE DEFERRAL. THERE WAS NOT ANY REF TO THE SLIDE IN THE ACFT MEL. THE CREW CONFERRED WITH COMPANY MAINT CTL; FLT DISPATCH; AND ASSISTANT CHIEF PLT ON DUTY FOR FURTHER GUIDANCE. AFTER REASSURANCES FROM ALL PARTIES THAT THIS WAS INDEED A LEGAL DEFERRAL; THE FLT WAS DISPATCHED AND FLEW TO SDF. SEVERAL DAYS LATER; MAINT CTL DETERMINED THAT THE SLIDE SHOULD HAVE BEEN TOTALLY REMOVED INSTEAD OF DEFERRED AS WAS THE CASE DURING OUR FLT AND RPTED THE DEV TO THE FAA.