IBM 6580 Displaywriter

The recent talk about two old Displaywriters being up for grabs got me to thinking about something that I've left hanging out in my "things to do" list. Does anyone know the internals of the Displaywriter file format? The disks themselves are SSSD with the first track being 26x128 byte sectors and the remainder 15x256. The encoding is EBCDIC for the text, obviously, but the structure has eluded me (or I've been just too lazy to figure it out). My notes from years ago say that it appears to be a two-level sort of affair, with the top level referring to documents and the second referring to pages within the documents--and then I became distracted and that's where my notes leave off. Does anyone know anything more about these things? I'd like to close this item out in my "things to do list'. Cheers, Chuck