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

box
Unix box

    1. A computer; especially in the construction "foo
   box" where foo is some functional qualifier, like "graphics",
   or the name of an operating system (thus, "Unix box",
   "MS-DOS box", etc.)  "We preprocess the data on Unix boxes
   before handing it up to the mainframe."  The plural
   "boxen" is sometimes seen.

   2. Without qualification in an IBM SNA site, "box" refers
   specifically to an IBM front-end processor.

   [Jargon File]

   (1994-11-29)