I think the "best" cases of No.1 status were the Flavio Briatore teams. I don't know, what was written in driver contracts at the time, maybe nothing. But I mean Flavio's overall approach and the way team operated. He seemed to be a big believer in the importance of Drivers Championship, compared to Constructors, and believer in a star driver, around who the team can be built.
Throughout years there have been too much noise/smoke coming from former #2 driver that they weren't really treated equally, compared to other teams, so this is also an additional consideration.