I have some rather old MO disks as well, but they are
made by Sony and are for
a 5 1/4" Magneto Optical Disk drive - an S501. They are very close
to 290 MBytes
on each side and are formatted for 512 bytes per sector.
If I recall correctly, one of the problems with the MO drives
was that there was little to no standardization of the formats or
disks for the drives. I've got an old Pinnacle MO drive stashed away
somewhere but no longer recall it's capacity.
Jeff
--
Collector of Classic Microcomputers and Video Game Systems:
Home of the TRS-80 Model 2000 FAQ File
http://www.geocities.com/siliconvalley/lakes/6757