A B727-100 FREIGHTER FO LEVELS OFF 300 FT HIGH WHEN THE RATE OF LEVELOFF IS ADMITTEDLY SLOWER THAN ANTICIPATED WHILE USING THE 'OLDER' AUTOPLT TRIM WHEEL. THE FO'S ALTIMETER WAS READING 150 FT LESS THAN THE CAPT'S; WHICH THEN TRIGGERED THE ACFT'S ALT WARNING AND THE ATC ALT WARNING.

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AS PER INSTRUCTED BY ZDC (FREQ 131.27) I WAS ABOUT TO LEVEL AT FL310. AT THE TIME; THE AUTOPLT WAS FLYING THE ACFT OR BETTER SAID; ENGAGED. DUE TO OLD EQUIP OR UNKNOWN REASONS; I WAS A LITTLE LATE IN LEVELING THE ACFT AT FL310 USING THE CTL KNOB ON THE CTR CONSOLE. BY THE TIME I HAD STOPPED THE ACFT MY ALTIMETER SHOWED AT FL311.5. HOWEVER; THE CAPT'S ALTIMETER SHOWED FL313. THIS ACTIVATED THE ALTIMETER AURAL SIGNAL WHICH IS ACTIVATED/DEACTIVATED AT APPROX +/-300 FT FROM THE ALT ENTERED ON THE ALT WINDOW. THE AIR DATA COMPUTER #1 WHICH RECEIVES DATA FROM THE #1 (CAPT'S) ALTIMETER SENDS THE SIGNAL (XPONDER) TO THE CTLR. SINCE THERE WAS A CLR NIGHT AND VISIBILITY WAS UNRESTR; NO ACFT WAS SEEN NEAR US AT THE TIME OF THE OCCURRENCE. ZDC DID NOT INQUIRE AT ANY TIME ABOUT OUR ALT. MY CORRECTIVE ACTION WAS TO DISENGAGE THE AUTOPLT AND LEVEL THE ACFT AT FL310 USING THE CAPT'S ALTIMETER. AT THIS TIME WE NOTICE THE DISCREPANCY BTWN ALTIMETERS WHICH HAPPENS TO BE BTWN PARAMETERS IN OUR FLT MANUAL. MY PERCEPTION IN RETROSPECT OF THIS SIT WAS THAT MAYBE I WAS LATE IN LEVELING THE ACFT DUE TO THIS BEING THE 4TH WORKING NIGHT ON THE BACKSIDE OF THE CLOCK WHICH IS ABOUT THE TIME THAT I START FEELING A WK'S WORK SOMETIMES. THE OTHER IS THE NONSTANDARDIZATION OF OUR B727 FLEET (COCKPIT LAYOUT) DUE TO ACQUISITIONS OF B727'S FROM DIFFERENT COMPANIES.