There is only one file type that uses the .LOP extension:
MasterCook Layout File
Stores the layout information for a Mastercook cookbook; created when a custom cookbook layout is saved.
If the LOP file is removed, an error message may appear when opening the file. To prevent the error, delete the associated files ending with ".mks," ".mlk," and ".rpt." Do not delete the ".mcf" and ".mrf" files, as these contain the actual cookbook data.
Updated: January 17, 2007