I do believe that you´ve accidently hit "the sweet spot" with your relatively limited research material, and are forming this theory on a collection of engines pre and post the mid 80´s, where Jensen changed their mold for 20 bases.
(I don´t know when they changed back again, but used to own a 2009 #20 with sharp corners on the base.)
My early 70´s 20 here, sits on the exact same type of base as Gil´s 1973 20R shown above.
Pretty big chance that the 20R setup was decommisioned, before the round corner bases was introduced, thus making it pretty hard to find a rounded corner version 20R.