What's inside a Simulink SLX file?

Simulink SLX files are just ZIP-compressed XML files.

So if you have mymodel.slx, you can just run

unzip mymodel.slx

and you will find files such as for example:

metadata/mwcoreProperties.xml  
metadata/mwcorePropertiesExtension.xml  
simulink/ScheduleCore.xml  
simulink/ScheduleEditor.xml  
simulink/bddefaults.xml  
simulink/blockdiagram.xml  
simulink/configSet0.xml  
simulink/configSetInfo.xml  
simulink/modelDictionary.xml  
simulink/plugins/DiagnosticSuppressor.xml  
simulink/plugins/LogicAnalyzerPlugin.xml  
simulink/plugins/NotesPlugin.xml  
simulink/plugins/SLCCPlugin.xml  
simulink/plugins/WebScopes_FoundationPlugin.xml  
simulink/stateflow.xml  
[Content_Types].xml     
_rels/.rels             
metadata/thumbnail.png  
simulink/_rels/blockdiagram.xml.rels  
simulink/_rels/configSetInfo.xml.rels  
simulink/codeDictionary.xml  
simulink/graphicalInterface.xml  

The most important file is simulink/blockdiagram.xml, which contains the actual model data. This file contains the model structure, including blocks, lines, and their properties.