The MS-DOS compression format uses the SZDD algorithm to compress a single file into another single file, replacing the last character in the filename with an underscore. It is implemented via the MS-DOS commands COMPRESS.EXE and EXPAND.EXE.
A DOS compressed file will be named file.**_, so a DOS compressed .exe will be named 'file'.ex_, a DOS compressed .jpg will be named 'file'.jp_.
https://www.cabextract.org.uk/libmspack/doc/szdd_kwaj_format.html