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

Intel 487SX
i487

    A version of the Intel 486DX microprocessor
   with an extra pin, for use in the coprocessor socket of an
   Intel 486SX system.  The 487SX provides the FPU which is
   missing in the 486SX.

   Although the 486SX is completely disabled when you install a
   487SX, the 487SX design requires that you leave the 486SX in
   your PC [why?], rather than use it elsewhere.  Intel admits
   that in some systems you can unplug the 486SX and fit a 487SX
   in its place but they don't guarantee that it will always
   work.

   See Intel 486.

   (1995-05-10)