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

Plug Compatible Manufacturer

    (PCM) A manufacturer of equipment that some other
   manufacturer's system can identify and work with.  The PCM's
   device replaces the original manufacturer's.

   Most PCMs competed with IBM.  PCM devices normally offer a
   cost-performance benefit over the original device.  For
   example, several PCM versions of the Direct-Access Storage
   Device IBM 3350 offered twice the storage and improved data
   access (dual port).

   Plug compatible devices include replacement CPUs, such as
   the Hitachi 7/90 series (which could be substituted for IBM
   3090 series processors), I/O subsystems, and dumb
   terminals like the IBM 3270.

   [Used outside mainframe market?]

   (2003-06-24)