DiskDoubler

DiskDoubler (DD) was a data compression utility for compressing files on the Apple Macintosh platform. Unlike most such programs, which compress numerous files into a single archive for transmission, DiskDoubler was intended to compress single files "in place" to save space on the drive. When such a file was opened, DiskDoubler would decompress the file before handing it off to the application for use. A later addition, AutoDoubler, added background compression, finding and compressing files automatically when the computer was idle.

File Identification
DiskDoubler files have no extension, they maintain the source file name and extension (if used).

DiskDoubler files have the magic number (hex values)  for the first 4 bytes.

The header will also include the Macintosh Type/Creator code for the original source file.

Links

 * Wikipedia article
 * Opening DiskDoubler files from Mac OS 9
 * Linux software which support decoding of DiskDoubler files