>>This is even older than that. I've found twenty pages with the lists of media descriptor byte values, and none of them remember this format. It was removed some time later, and if I didn't have these 30-40 disks formatted so, I would have sworn the format never existed :).
>
>Check this: http://thestarman.pcministry.com/asm/mbr/DOS50FDB.htm#BPB
>Apparently the media descriptor byte is F9 for both 720K and 1200K (ambiguity which forced the 720K out of standard), but the number of sectors per track is different. I wonder what would happen if you could change both offset 15 (MDB) value to F9, and offset 18-19 (SPT) to 0900. Obviously, if you can't even access the disk, this is a moot point.

This worked fine under DOS 3.1, and I think there was still a bootable floppy of the sort in the deck. So I may just boot one and see what gives. Perhaps build a VM from it :). More fun for next winter.