THE WRONG SERIAL NUMBER FLAP IS DISCOVERED TO BE INSTALLED ON THE EMB-120.

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YAW DAMP DEFERRED ON ACFT AND PREVIOUS CONTROLLABILITY PROBS RPTED BY PREVIOUS CREW -- FLEW 1 ROUND TRIP WITHOUT INCIDENT; BUT DURING WALKAROUND NOTED R FLAP APPEARED OUT OF ALIGNMENT WITH TRAILING EDGE OF WING. UPON FURTHER INSPECTION; ROLLER WAS FOUND TO BE CRACKED AND MAINT WAS CALLED. MECH DETERMINED THAT INCORRECT SERIAL NUMBER FLAP HAD BEEN INSTALLED; GNDING ACFT UNTIL SUITABLE REPLACEMENT COULD BE OBTAINED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE TYPE WAS AN E-120-30 PAX TRANSPORT. THE LOCATION WAS ON THE RAMP AT ATLANTA. THE FO CALLED TO THE CAPT'S ATTN THE VARIANCE IN ALIGNMENT OF THE FLAP GAP. THE FLAP WAS FROM ONE OF THE FIRST 5 OR 6 E-120 MADE AND THE RPTR SAID THOSE 5 OR 6 EARLY AIRPLANES WERE ALMOST HAND MADE AT THAT TIME AND THEIR PARTS ARE NOT INTERCHANGEABLE WITH THIS NEWER AIRPLANE. THE CRACKS ARE DIFFICULT IF NOT IMPOSSIBLE TO SEE WITHOUT MAINT ASSISTANCE. BUT; THE OUT OF ALIGNMENT CAN BE SEEN. THE RPTR SAID THIS WOULD PROBABLY NOT HAPPEN WITH ANOTHER AIRLINE BECAUSE THIS AIRLINE HAS 60 OF THESE E-120 AND THE RPTR BELIEVES ALL OF THE FIRST 5 OR 6 THAT WERE HAND MADE.