A DC9 WAS REPAIRED BUT WRONG PART NUMBER ENTERED IN LOG SIGNOFF.

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DC9 ACFT ARRIVED AT GATE WITH FOLLOWING WRITE-UP. COMPUTER FLAG; FO ADI. FOUND FO STEERING COMPUTER MISSING FROM RACK. CHKED ACFT COMPARTMENT TO LOCATE MISSING PART. COULD NOT BE FOUND. INSTALLED SVCABLE PART; OPS CHKED NORMAL. AFTER THIS EVENT; I FOUND OUT ACFT HAD BEEN IN HVY MAINT AND FLOWN 1 LEG. UPON DISCUSSION WITH TECH CTR; I FOUND OUT THE SERIAL NUMBER OF THE PART THAT WAS SUPPOSED TO BE IN THE ACFT. AFTER TALKING WITH TECH CTR. I USED THIS SERIAL NUMBER ON ACFT LOG SHEET TO MAKE IT EASIER TO ENTER INTO DIRECT DATA ENTRY ON COMPANY COMPUTER SYS. PLEASE NOTE I HAD NO INTENTION OF DISPATCHING THE ACFT WITH THIS PART MISSING. MY MISTAKE WAS USING THE SERIAL NUMBER OF THE PART THAT WAS SUPPOSED TO BE IN THE ACFT ON THE LOG SHEET. I'M SO USED TO WRITING 'REMOVED AND REPLACED' IN THE LOG SHEET SIGN-OFFS THAT IT TRAPPED ME. THINKING BACK; I SHOULD HAVE SLOWED DOWN AND ASSESSED THE SIT. THE PROPER WAY TO DO IT WOULD BE TO JUST INSTALL THE PART ONLY AND LEAVE THE OFF SERIAL NUMBER BLOCK ON LOG SHEET BLANK.