ALTDEV ALT UNDERSHOT. ACR CREW FLYING AN OLD B737-200; EQUIPPED WITH A 3 POINTER (NON DIGITAL) ALTIMETER; MISREADS THE ALT BY 10000 FT HIGH.

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OUR ACFT WAS AN OLD ORIGINAL BOEING 737-200 WHICH WAS EQUIPPED WITH THE OLD STYLE POINTER ALTIMETERS THAT ARE SO NOTORIOUS FOR MISREADS AND DID NOT HAVE A DIGITAL/DRUM TYPE OF ALTIMETER. AS WE APCHED GROAN INTXN; BEFORE THE TURN; THE FO MISREAD THE ALTIMETER AND; THINKING SHE WAS AT 10000 FT; LEVELED OFF AT FL200. I DID NOT CATCH THIS AS OUR COCKPIT WORKLOAD WAS HIGH AT THIS POINT. WE WERE IN WX AND WE HAD A SHORT DISCUSSION ABOUT USING THE ENG ANTI-ICE; WHILE VERIFYING; RETUNING AND IDENTING THE SGD VOR AND THE INBOUND SGD 107 DEG RADIAL; VERIFYING THE ATIS WINDS FOR THE LNDG FLAP SETTING; ACCOMPLISHING THE APCH DSCNT CHKLIST; ETC. ALSO CONTRIBUTING TO THE ERROR WAS THE IMPORTANT FACTOR OF THE CREEPING FATIGUE OF THE LONG DUTY DAYS OF THIS TRIP. THE PREVIOUS DAY WE HAD FLOWN A 12 HR DUTY DAY FOLLOWED BY A SHORT LAYOVER; DUE TO SOME DELAYS; WHICH IS NEAR THE NEW DENVER ARPT REQUIRING 1 HR OF TRAVEL TIME. AT THE TIME OF THIS INCIDENT; WE WERE FLYING THE 10 1/2 HR OF A SCHEDULED 14:26 HR (INCLUDE RPT TIME) DUTY DAY. IT WAS VERY SHORTLY AFTER LEVELING OFF CTR CALLED ASKING IF WE WERE AT 10000 FT. I GLANCED UP AT THE ALTIMETER AND FOR THE MOMENT ASSUMED AND THOUGHT THAT WE WERE AT 10000 FT. I STARTED TO SAY YES WHEN I REALIZED THAT THE ALTIMETER INDICATED 20000 FT; NOT 10000 FT; AND I PUNCHED THE TCASII ALT BUTTON AND VERIFIED THAT WE WERE INDEED AT 20000 FT. AS I SAW THAT WE WERE APCHING 15 DME FROM LOCKE; I CALCULATED THAT WE COULD MAKE AN IMMEDIATE DSCNT AND CROSS LOCKE AT THE REQUIRED 10000 FT. I IMMEDIATELY RESPONDED WITH THE STATEMENT THAT WE WERE AT FL200 AND THAT WE WERE INITIATING AN IMMEDIATE DSCNT TO 10000 FT. BEFORE WE COULD START THE PUSHOVER THE CTLR CLRED US TO MAINTAIN FL200 AND GAVE US A HDG TO THE R; OFF THE AIRWAY. HE ADVISED THAT HE WAS GOING TO VECTOR US BACK AROUND TO REINTERCEPT THE ARR FLOW. HE FURTHER VECTORED AND DSNDED US WITHOUT FURTHER INCIDENT OR COMMENT. HOWEVER; WHILE HANDING US OFF TO BAY APCH HE REQUESTED THAT WE CALL CTR UPON LNDG TO DISCUSS A POSSIBLE VIOLATION AND GAVE US A PHONE NUMBER. WE DID NOT CROSS LOCKE INTXN DUE TO THE NEW CLRNC AND COULD HAVE MADE THE XING AT LOCKE AT 10000 FT. NO TCASII WARNINGS OR ALERTS RECEIVED. NOT ONE ADVISEMENT OF ANY OTHER ACFT IN OUR AREA WAS GIVEN BY ATC AT ANY TIME. AFTER LNDG I CONTACTED CTR AND SPOKE WITH THE AREA COORDINATOR FOR ZOA AND HE ADVISED THAT THE CTR HAD RECEIVED AN AUTOMATIC WARNING INDICATING A POSSIBLE CONFLICT WITH A SFO DEP AND WAS THEREFORE REQUIRED TO RPT THE INCIDENT. HE GAVE NO FURTHER DETAILS REGARDING THE ALLEGED 'VIOLATION.' AS HE DID NOT ELABORATE ON WHAT EXACTLY THE VIOLATION WAS OTHER THAN THE CTR'S INTERNAL WARNING; I CANNOT SPECULATE ON CONTRIBUTING OR PREVENTIVE FACTORS BEYOND THOSE SET FORTH ABOVE AT THIS TIME.