On Sun, 10 Sep 2023, Chuck Guzis via cctalk wrote:
  There are 40 track derivatives; used for word
processing, particularly
 on some Brother models.  No big deal; when reading those, one simply
 double-steps a "normal" drive.  In any case, as far as I recall, they
 all used Brother's proprietary GCR encoding.  I've processed a couple
 hundred of those. 
With the exception, the Epson Geneva PX8 40 track 3.5" was MFM
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Grumpy Ol' Fred                     cisin(a)xenosoft.com