Prefab, Apply button missing. Only see Open and Select

I'm using Unity 2019.2.3f1 on Ubuntu.

When clicking an instance of a Prefab in my scene, the Inspector shows this:



I expected to see Select, Revert, and Apply as the button options next to "Prefab", but in that picture there's only Open and Select.

Am I not viewing the right thing, so my expectations are off?

Perhaps Overrides is relevant, but after playing around with it I'm still unclear on how it works.

Thanks to anyone who has any advice.

 

Editing a Prefab via its instances