FLC OF AIR CARRIER LTT ACFT OVERSHOT DSCNT CLRNC LIMIT ALT RESULTING IN NO CONFLICT.

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I WAS FO; PF ON AIR CARRIER ACFT FLT FROM GSO TO RDU ON MAY/SUN/92. DEPARTED GSO ON SCHEDULE AND CLBED TO 7000 MSL. HANDED OFF FROM GSO DEP TO RDU APCH (132.35) WAS GIVEN DSCNT TO 5000 MSL BUT DSNDED TO 4000 MSL INADVERTENTLY. THE ONLY ALT REMINDER THAT WAS IN USE IN THIS ACFT WAS MY ADF'S STANDBY FREQ WHICH I DIAL TO THE ASSIGNED ALT. THIS PARTICULAR ACFT DID NOT HAVE ANY PROVISIONS FOR THE CAPT TO HAVE AN ALT REMINDER. ALL OTHER ACFT IN THE FLEET HAVE A ROLL-TYPE ALT REMINDER ON CAPT'S SIDE. THE ADF REMINDER ON MY SIDE IS POSITIONED SO IT IS DIFFICULT TO SEE DUE TO THE POS OF THE YOKE. I FEEL THAT HAD THE ACFT BEEN INSTALLED WITH AN AUDIBLE ALT ALERTING SYS; THE DEV WOULD NOT HAVE OCCURRED. ANOTHER FACTOR IS CREW FATIGUE -- WE WERE ON THE LAST LEG OF A 14 HR DUTY DAY WHICH HAD BEEN THE LAST DAY OF 6 DAYS IN A ROW. NO CONFLICT HAD OCCURRED; AND THE CAPT ASKED THE APCH CTLR IF ANY SEPARATION PROBLEMS HAD OCCURRED. HE REPLIED THAT THERE WAS NO SEPARATION PROBLEM AND THAT 4000 MSL WAS 'OK.'