SET MILLIBARS VERSUS INCHES ON THE CAPT ALTIMETER; RESULTING IN OFF ALT 600 FEET.

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ALTITUDE CLRNC WAS FL240. AT 17500 FT I RESET THE CAPT'S ALTIMETER TO 29.92 AND VERBALLY CALLED '29.92' AS PER COMPANY PROCEDURES. SHORTLY AFTER LEVEL OFF; CENTER ASKED US TO VERIFY OUR ALTITUDE. IT WAS THEN NOTICED THAT MY (CAPT) ALTIMETER READ FL240; WHILE THE FIRST OFFICER'S READ FL246. CENTER IMMEDIATELY CLRED US TO CLIMB TO FL290 AND WE THEN CONTINUED OUR CLIMB. THE PROBLEM WAS CAUSED BY MY SETTING 992 IN THE MILLIBAR WINDOW ON THE CAPTAIN'S ALTIMETER BY MISTAKE; INSTEAD OF 29.92 IN THE INCHES WINDOW. A CONTRIBUTING FACTOR TO THIS WAS THE FACT THAT THIS PARTICULAR ACFT IS A NONSTANDARD VARIANT IN THE COMPANY FLEET. AMONG SEVERAL DIFFERENCES IS THE CAPTAIN'S ALTIMETER WHICH HAS THE MILLIBAR AND IN HG WINDOWS REVERSED FROM MOST OF THE REST OF THE FLEET. (THE USUAL POSITION IS MB-LOWER LEFT; AND IN HG-LOWER RIGHT. THIS ACFT HAS MILLIBARS-LOWER RIGHT AND IN HG LOWER LEFT). ALSO; THE 'MB' AND 'IN HG' CAPTIONS ABOVE THE RESPECTIVE WINDOWS ARE MISSING ON THIS PARTICULAR ALTIMETER. STANDARDIZATION OF INSTRUMENT FACE PRESENTATIONS WOULD HELP PREVENT THIS TYPE OF INCIDENT; I WOULD THINK. WE TEND TO DO THINGS FROM HABIT PATTERNS INSTEAD OF ACTUALLY NOTICING DETAILS WHICH MAY DIFFER FROM THE NORM.