I was taught assembly at the job by an old assembly guru. His first statement to me was something along the lines of: "we'll use an assembler to start with, but it doesn't generate very good code; so, once you're comfortable, we'll hand assemble our machine code".
Was he just messing with you? What's a case where assembly is not isomorphic to machine code?
Nonsense. Octal, not hex.
(Not totally a joke. My mother wrote in octal before she got an assembler.)