ALT OVERSHOT BY DC9 FLC DURING DSCNT WHILE ON DOWNWIND VECTOR FOR APCH. WHEN THE ALT ALERTER SOUNDED; THE PF MISTAKENLY CALLED 6600 FT FOR 5600 FT WHEN THE ACFT WAS ACTUALLY DSNDING THROUGH THE ASSIGNED ALT OF 5600 FT. THE PNF WAS WATCHING TFC AHEAD WHICH WAS UNABLE TO MAINTAIN MATCHING SPD AND DISTANCE. CLRNC TO MAINTAIN 5600 FT WAS ASSIGNED FOR SEQUENCING.

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WE WERE BEING VECTORED FOR ILS RWY 18 AT CLT. WE WERE ON A R DOWNWIND DSNDING TO 4600 FT MSL ON APPROX A 030 DEG HDG. I WAS LISTENING TO THE CTLR TALKING TO AN ACFT AHEAD OF US. HE ASKED THAT ACFT TO FLY AT 190 KTS TO MATCH OUR SPD (WE WERE FOLLOWING HIM). THE PLT RESPONDED THAT THEY WERE UNABLE TO MAINTAIN 190 KTS. SO THE CTLR TOLD US TO MAINTAIN 5600 FT MSL. I READ BACK '5600.' THE FO (WHO WAS THE PF) SET 5600 FT IN THE ALT ALERTER WINDOW; AND I POINTED TO IT AND VERIFIED IT. A FEW SECONDS LATER THE ALT ALERTER CHIME WENT OFF AND THE FO RESPONDED '6600 FT FOR 5600 FT.' I WAS WATCHING THE TFC ON TCASII AND NOTICED THAT WE WERE STILL CLOSING ON HIM AND DSNDING DOWN ON HIM. THIS CONFUSED ME BECAUSE I THOUGHT THAT AT 5600 FT MSL WE WOULD BE 1000 FT ABOVE HIM. IT WAS THEN THAT I DOUBLE CHKED MY ALTIMETER; IT READ APPROX 5000 FT AND DSNDING. I REALIZED THEN THAT THE FO HAD MISREAD OUR ALT AND I HAD FAILED TO CATCH THE MISTAKE. I SAID WE ARE TOO LOW CLB BACK UP TO 5600 FT. SO THE FO ADDED PWR AND CAUGHT OUR ALT AND SAID; 'JUST MAINTAIN YOUR CURRENT ALT AND TURN L TO 010 DEGS.' THERE WAS NO ACTUAL CONFLICT WITH THE OTHER ACFT. I BELIEVE THAT THIS ERROR WAS A DIRECT RESULT OF MISREADING THE ALTIMETER. THE ALTIMETER IN OUR ACFT IS THE OLD BARREL TYPE WITH A SINGLE HUNDREDS FOOT POINTER. THE PROB IS YOU REALLY HAVE TO THINK ABOUT WHAT THOUSANDS YOU ARE ON AND IF YOU ARE ABOVE OR BELOW IT. THE NEWER TYPE WITH THE ENTIRE ALT READ OUT IN THE BARREL (THOUSANDS; HUNDREDS; TENS) ELIMINATES THIS PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: BOTH SIDES OF THE COCKPIT HAVE THIS TYPE OF ALTIMETER. IN A QUICK GLANCE; IT'S CONFUSING TO READ. THIS IS ONE OF THOSE TIMES THAT THE CAPT (RPTR AND PNF) FAILED TO RECOGNIZE THE DEV BECAUSE OF THE FOCUS THAT READING THIS PARTICULAR ALTIMETER REQUIRES AND THE CONFLICTING DEMANDS FOR HIS ATTN. THE CAPT WAS CONCERNED WITH THE TFC AND SPACING AND RELYING HEAVILY UPON THE FO TO FLY THE ACFT. SUPPLEMENTAL INFO FROM ACN 328357: FACTORS THAT CONTRIBUTED TO THE PROB WERE PLT FATIGUE (SHORT OVERNIGHT) AND A CAPT UNDER HIGH WORKLOAD. I WAS THINKING DSNDING; NOT CLBING TO AN ALT.