type of aircraft

Date:         05 Oct 98 00:26:56 
From:         hasse@iphcip1.physik.uni-mainz.de
Organization: Johannes Gutenberg-Universitaet Mainz, Germany
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What is the difference between a  Boing 737-200 Advanced  and  a
Boing 737-275 Advanced (or a 727-247 (F) Advanced and a 727-260 (F)
Where can i found some information about the second number of the
aircraft description? Or is there a simple system behind the number?