There is only one file type that uses the .MNS extension:
AutoCAD Interface Settings File
User interface settings file used by AutoCAD, a 2D and 3D design and documentation tool; stores ASCII text properties that define how user interface elements are displayed; can be used for customizing the AutoCAD workspace layout.
AutoCAD supported MNS files prior to version 2006. MNS files are the same as .MNU files, but cannot contain special formatting or comments.
NOTE: Autodesk replaced MNS files by the .CUI file format in AutoCAD 2006. CUI files were later replaced by .CUIX files in AutoCAD 2010. MNS files can be converted to CUIX files using the Transfer tab of the CUI command in AutoCAD 2010.
Updated: March 25, 2010