Starting from OSM snapshot 2011-05-25, delta files were separated from the maps files.
All delta files for a particular OSM snapshot are stored in a separate folder named after source and target snapshots dates and each delta file is named after region name. Region name is followed by the size (in bytes) of the source file used for delta file generation.
Structure of folders for map files is unchanged. GMAPSUPP.IMG files are stored in folders named after particular region.