I was very careful to say "automatic Autavias" because I've seen 2446s with polished casebacks too and I'm not sure on the original finish :)

Poly back Carreras I've seen in the metal have all had at least vestigial traces of linear brushing, so I lean towards them having been brushed ex-factory. With photos it's sometimes harder to tell, especially after 47 years of wear!

Slightly off-topic perhaps, but take a look at the back of this 2456 belonging to Arno:

Now that's a beautiful caseback!