A C421C ACFT ON DEP AND INSTRUCTED TO REMAIN BELOW CLASS B AIRSPACE; THE BASE OF WHICH IS 4500 FT. FLC EITHER HAD A MODE C ERROR OR STARTED CLB ABOVE 4500 FT TOO EARLY AND CLIPPED THE EDGE OF CLASS B.

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A C421C DEPARTED FROM NORTH LAS VEGAS ARPT APPROX XX10 LCL TIME UNDER VMC CONDITIONS AND WAS GIVEN A DISCRETE XPONDER CODE FOR CLASS C SVC TO CHINO; CA; AT 14500 FT. DEP INSTRUCTIONS FROM NORTH LAS VEGAS INCLUDED 'FLY HDG OF 220 DEGS AND MAINTAIN OUTSIDE CLASS B AIRSPACE.' FOLLOWING TKOF; WE FLEW HDG 220 DEGS AND CLBED TO AN INDICATED ALT OF 4500 FT MSL (THE OVERLYING AIRSPACE IN THE AREA HAS A FLOOR OF 4500 FT). AFTER CLRING THE OVERLYING CLASS B AIRSPACE; WE CONTINUED VFR CLB AND WERE XFERRED FROM NORTH LAS VEGAS TWR TO LAS VEGAS APCH CTL. UPON CONTACTING LAS VEGAS APCH; THE APCH CTLR INFORMED US THAT WE HAD ENTERED CLASS B AIRSPACE AT 4700 FT (200 FT ABOVE THE FLOOR OF THE SPACE IN THAT AREA). THE APCH CTLR INSTRUCTED US TO PHONE THE LCL TRACON SUPVR AND THEN PROVIDED US WITH A HEADING AND CLRED US TO 14500 FT. IT IS POSSIBLE THAT THE 200 FT INCURSION INTO CLASS B AIRSPACE OCCURRED AS A RESULT OF A +/-200 FT DIFFERENCE BTWN OUR ALTIMETER AND ITS MODE C. ALTERNATIVELY; WE MAY HAVE RESUMED CLB AND 'CLIPPED' THE VERY EDGE OF THE CLASS B AIRSPACE UPON RESUMING CLBOUT. THIS SIT COULD BE AVOIDED BY LEVELING OFF AT A LOWER ALT WHILE DEPARTING UNDER THE OVERLYING CLASS B UNTIL XFERRED FROM NORTH LAS VEGAS TWR TO LAS VEGAS APCH AND RESUMING CLBOUT ONLY AFTER WELL CLR OF THE CLASS B AIRSPACE OR UNTIL CLRNC THROUGH CLASS B AIRSPACE IS RECEIVED.