The Free On-line Dictionary of Computing (30 December 2018):

Fault Tolerant Unix
FTX

    (FTX) Stratus's own Unix System V
   Release 4 multiprocessor operating system.  In 2016, FTX
   is supported but no longer developed.

   FTX was one of three operating systems supplied by Stratus on
   their hardware, the other two, HP-UX and VOS, were the
   more common choices, FTX was only sold on an exceptional
   basis.

   Early FTX 3.x releases used an in-house virtual disk layer
   (VDL) driver, but later releases switched to a version of
   Veritas VxVM.

   FTX supported many of the proprietary communications boards
   (ISDN, serial, parallel, X.25, etc.).

   (http://www.openpa.net/systems/stratus_continuum.html)

   (1998-07-06)