It's not based on V6, although it was basically created as a clone of it (and on the PDP-11, no less - the x86 versions of Coherent until 4.x were basically just minimal ports that kept the PDP-11's classic 16-bit flavour). It was a very very good clone, enough that a lawsuit was threatened and Dennis Ritchie himself got pulled into checking that it wasn't derived from any UNIX source - see dmr's comment at https://groups.google.com/g/alt.folklore.computers/c/_ZaYeY4...