The Jargon File (version 4.4.7, 29 Dec 2003):

PDP-20
 n.

    The most famous computer that never was. PDP-10 computers running the 
    TOPS-10 operating system were labeled ?DECsystem-10? as a way of
    differentiating them from the PDP-11. Later on, those systems running 
    TOPS-20 were labeled ?DECSYSTEM-20? (the block capitals being the result
    of a lawsuit brought against DEC by Singer, which once made a computer
    called ?system-10?), but contrary to popular lore there was never a
    ?PDP-20?; the only difference between a 10 and a 20 was the operating
    system and the color of the paint. Most (but not all) machines sold to run
    TOPS-10 were painted ?Basil Blue?, whereas most TOPS-20 machines were
    painted ?Chinese Red? (often mistakenly called orange).


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

PDP-20

   The most famous computer that never was.  PDP-10 computers
   running the TOPS-10 operating system were labelled
   "DECsystem-10" as a way of differentiating them from the
   PDP-11.  Later on, those systems running TOPS-20 were
   labelled "DECSYSTEM-20" (the block capitals being the result
   of a lawsuit brought against DEC by Singer, which once made a
   computer called "system-10"), but contrary to popular lore
   there was never a "PDP-20"; the only difference between a 10
   and a 20 was the operating system and the colour of the
   paint.  Most (but not all) machines sold to run TOPS-10 were
   painted "Basil Blue", whereas most TOPS-20 machines were
   painted "Chinese Red" (often mistakenly called orange).

   [Jargon File]

   (1994-12-21)