It's clear that the authors were either inspired by the PDP-11 ISA and/or designed the language to make porting UNIX from PDP-11 Assm to it easier.
replyUNIX for PDP-7 in Assm -> UNIX for PDP-11 in Assm -> UNIX for PDP-11 in C
https://news.ycombinator.com/item?id=42644851
and