ALTDEV IN DSCNT. ACFT MODEL DIFFERENCES IN AUTOMATION BTWN B737-200 AND B737-300.

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IN A DSCNT TO 26000 FT MSL FO (FLYING) WENT BELOW ASSIGNED ALT BY ABOUT 1000 FT (25000 MSL). SUPPLEMENTAL INFO FROM ACN 317248: THIS WAS THE 4TH DAY OF A 4 DAY TRIP. THE FIRST 3 DAYS WE FLEW ONLY B737-300'S WHICH AUTOMATICALLY LEVEL OFF WITH THE SELECTED ALT IF AUTOPLT IS IN USE. THIS DAY WE WERE FLYING A B737-200 WHICH DOES NOT AUTOMATICALLY LEVEL OFF. THE SUN WAS AT AN ANGLE THAT WAS REALLY DISTRACTING. I HAD JUST TURNED SLIGHTLY TO STAY ON THE RADIAL AND AS A RESULT THE SUN HAD CHANGED POS IN WINDSCREEN. I WAS TRYING TO ADJUST SUN SCREEN AND DID NOT REALIZE THAT WE WERE DSNDING THROUGH OUR CLRNC ALT. WE DSNDED APPROX 1000 FT BELOW (FL250) WHEN CTR ASKED OUR ALT. AT THIS POINT WE CLBED QUICKLY BACK TO FL260. VERY SHORTLY THEREAFTER WE WERE CLRED TO FL240. THIS TRIP WAS THE 4TH IN MY FIRST MONTH FLYING WITH TYPE. I'M STILL WORKING HARD TO MAKE SURE I AM ACCOMPLISHING MY DUTIES; AS LAID OUT BY THE OPS MANUAL; IN A TIMELY AND CORRECT MANNER. THE CTLR DID NOT INDICATE ANY ALARM NOR WAS THERE ANY OTHER ANNOUNCED OR TCASII TFC IN THE AREA AT THE TIME. WHEN WE ANNOUNCED WE WERE CLBING BACK TO FL260; THE CTLR ANSWERED; ROGER. I THINK THIS INCIDENT WAS A RESULT OF A COMBINATION OF SEVERAL FACTORS. FIRST WAS MY LACK OF EXPERIENCE IN THIS TYPE ACFT. SECOND WAS THE CHANGE OF MODELS FROM B737-300 TO B737-200 WITHOUT THE AUTO LEVEL OFF FEATURE. THIRD WAS THE DISTR OF THE SUN. AND FINALLY; I WAS TRYING TO STAY ON THE DSCNT SCHEDULE REQUIRED OF THE B737 ON THE MADWIN 3 ARR AND PERCEIVED MYSELF BEHIND THE CORRECT DSCNT GRADIENT. SO I WAS DSNDING AT A FAIRLY QUICK RATE. I AM NOT SURE WHY NEITHER OF OUR 2 MAN CREW NOTICED THE DSCNT THROUGH ASSIGNED ALT. THEREFORE; OTHER THAN BECOMING MORE AWARE AND VIGILANT AS AN INDIVIDUAL PLT ABOUT ALT ASSIGNMENTS; I DO NOT HAVE ANY OTHER SUGGESTIONS TO PREVENT THIS TYPE OF MISTAKE.