This might give a clue.  The tape I have has the
following on the label.
                Zane
 IBM
 MT
 Composer
 K111
 Load...262
 Precon Check 
WHOA ... those machines are SO RARE.  In fact I haven't EVER seen a Mag
Tape Selectric Typewriter or Mag Tape Selectric Composer ... and the
Composer was made in much smaller quantities than the MT/ST.  (I do have a
Composer, an Electronic Selectric Composer or ESC, and a Mag Card
Selectric Composer or MC/SC ...)
I believe the MT/ST was the first significant word processing device, and
I would love to find one, but I'm not sure any exist any more.  If only
one ever turned up, I think it should go to the Computer History Museum.
Does anyone know of one?
Norm