This line indicates a PalmDOC meant to be opened with EZreader (Handheldmed or Intellectual Reserve), an old palm doc reader. These files are highly compressed. Tom Welch, the original developer of EZreader has expressed a willingness to have a converter developed for these old files. See here:
http://tech.lds.org/forum/showthread.php?t=1887
I personally have many books in this format and would love for Calibre to be able to convert them. Please consider it.