ShrinkIt

A NuFX archive is an archive [file format] that supports lossless data compression. It is usually given the file extension SHK; SDK may also be used to specify that the archive describes the entire contents of a disk.

History

Andy E. Nicholas designed the format as an improved replacement for the Binary II format, adding compression. He introduced it alongside a pkzip-like program called ShrinkIt for Apple II systems, which he later migrated to the Apple IIGS as GS-Shrinkit.
The specification was republished by Apple in its File Type Notes.