Displaywriter 8" filesystem?

I've imaged a little stack of Displaywriter 8" disks and am starting in on filesystem organization. With an EBCDIC filter I can see the data plain as day. I've also got experience stitching together the DisplayWriter files that come from the DOS-based system, so if I could get the file allocation scheme figured out and lift off whole files from these disk images, I bet the same conversion code would work on the resulting data. I've browsed the manuals on Bitsavers, but of course IBM never intended their users to actually look at the bytes on the disk. So none of the information was geared towards revealing the file allocation secrets. Is there any reference material out there to help me decode (or find) the file allocation structures? - David