PDP-11/20 vd one that just says pdp 11 what are the date differences?? OEM?

COURYHOUSE at aol.com COURYHOUSE at aol.com
Mon Feb 22 02:31:12 CST 2016

or maybe initially  they  figured to use letters after  the  11.... example 
  pdp-11/a pdp-11/E
then  once out   decided to  use    numbers as a dub designator 

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