StoneCracker
From Just Solve the File Format Problem
StoneCracker is a compressed file format produced by the Amiga packer program StoneCracker.
Identification
For version 3 and above, files begin with one of the following ASCII identifiers:
S300 S310 S400 S401 S403 S404
Earlier versions are more difficult to identify. There's an attempt in Ancient's source code.