Unlike the Chronomatic-powered Carreras, which changed from a radial to a linear brushing late on:

I think the automatic Autavias were fairly consistent in sticking with the linear brushed finish. At least until the PVD models came along with their more complex backs:

There may be some more variation that an Autavia collector could put us straight on, but I don't think that will include a straightforward polished back - this is after all by far the easiest way of refinishing a case and caseback and that's why we see it relatively often, people taking that convenient shortcut.