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

[COURYHOUSE at aol.com]

Then  there is  this  information.....

PDP-11/15
 

 (http://gunkies.org/wiki/PDP-11/15#column-one)  
(http://gunkies.org/wiki/PDP-11/15#searchInput) 
 

This is the OEM version of the _PDP-11/20_ 
(http://gunkies.org/wiki/PDP-11/20) .