While extracting a tarball, GNU/tar told me:

Ignoring unknown extended header keyword `SCHILY.dev'
Ignoring unknown extended header keyword `SCHILY.ino'
Ignoring unknown extended header keyword `SCHILY.nlink'

Hm, what does that remind me of? Ah, star! Digging in the bits still left on BerliOS, there seems to be something what looks like a POSIX proposal:

$ cat star/README.posix-2001
[...]
        Star supports the following fields in the extended header:
[...]
        Vendor unique:
        "SCHILY.devmajor" "SCHILY.devminor"     (create/extract)

        In -dump mode (a preparation for incremental dumps) star archives:

        "SCHILY.dev"            The field stat.st_dev   - the filesys indicator
        "SCHILY.ino"            The field stat.st_ino   - the file ID #
        "SCHILY.nlink"          The field stat.st_nlink - the hard link count
        "SCHILY.filetype"       The real file type      - this allows e.g.
                                                              socket/door

Wow. Alas, these headers remain unrecognized in GNU/tar, hence the (benign) warnings.