unix developed on 11/20 with 20 on panel or machine that just said pdp/11?

[Warren Toomey]

According to this page that Dennis Ritchie wrote, the original PDP-11
they used was indeed an 11/20 but it was before there were PDP-11 model
numbers:

https://www.bell-labs.com/usr/dmr/www/picture.html

And, of course, the PDP-7 Unix development came before the PDP-11 version :)

Cheers, Warren