On Wed, Sep 21, 2022 at 4:09 PM Rich Salz <rich.salz@gmail.com> wrote:
Recall also that the AT&T code was protected by "trade secret" claims, so that and the Berne convention meant a copyright on each file wasn't needed. As the copyright moved to being used as a license, doing so became more important.

Indeed. The lack of copyright notices prior to Berne was also a big problem for enforcing copyright claims, which is why most companies relied on trade secrets prior to the mid 80s. It's a big reason, I think, the settlement punted on trade secret protections, but made a big deal about respecting each other's copyrights (despite preliminary rulings casting doubt in that area).