mkextunpack(8) on macOS
The text 155f584c0a42, as Mac OS X 10.5.8, Mac OS X 10.5.7, Mac OS X 10.5.6, Mac OS X 10.5.5, Mac OS X 10.5.4, Mac OS X 10.5.3, Mac OS X 10.5.2, Mac OS X 10.5.1, Mac OS X 10.5 shipped it.
extracts the contents of a multikext (mkext) archive