A B737-300 WAS DISPATCHED WITH THE WRONG CAPT'S INST COMPARATOR ANNUNCIATOR INSTALLED.

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WHILE PERFORMING A MAINT CHK ON A B737-300 ON FEB/XX/98; ONE OF THE DISCREPANCIES I FOUND WAS THAT THE CAPT'S AND FO'S INST COMPARATOR ANNUNCIATORS HAD THE HDG LIGHTS INOP. AFTER GETTING THE PART NUMBER OFF THE OLD PART; I CHKED THE COMPUTER FOR AVAILABILITY AND SAW WE HAD ONE; WENT TO THE STOCK ROOM AND INADVERTENTLY PICKED UP THE WRONG PART. I RELIED ON MEMORY OF ITS LOCATION AND LABEL ON THE SHELF AND DID NOT REALIZE THAT IT WAS AN ANNUNCIATOR FOR A B737-200. I FAILED TO RECHK THE PART NUMBER AND INSTALLED THE PART ON THE CAPT'S SIDE. I TESTED THE ANNUNCIATOR AND IT TESTED GOOD. I THEN PLACED THE FO'S COMPARATOR ON MEL AND DOWNGRADED THE ACFT TO CAT I STATUS VIA AN MEL. THE NEXT NIGHT WHEN I RPTED FOR DUTY; THE STOCK CLERK INFORMED ME THAT THE PART ON WAS NOT THE SAME AS THE PART OFF. I IMMEDIATELY CONTACTED MAINT CTL TO GET THE MISTAKE CORRECTED. THE STATION HAD ALREADY REPLACED THE FO'S SIDE AND WE HAD TO MEL THE CAPT'S SIDE AND THE ACFT REMAINED ON CAT I STATUS. THE ACFT WAS TO HAVE THE INCORRECT ANNUNCIATOR REPLACED ASAP. IN RETROSPECT; THE PROB COULD HAVE BEEN AVOIDED HAD I TAKEN THE TIME TO VERIFY THE PART NUMBER AND NOT RELIED ON MY MEMORY OF STOCK ROOM LOCATION.