The pilot just supports the case, but I shouldn't have to tell you that. It doesn't make a difference if it's indicated inside or out side. I used RCBS, and other tools to indicate and they all measure the same with the exception of the ball mic. It's not that complicated Mike.No, a pilot does not provide true spot measure like a ball anvil.
This matters (functionally) because necks taper in thickness, including a donut area thickness. So with a pilot, you would read an average thickness from shoulders to mouths, rather than actual thickness at each chosen datum.
You don't want to add that thickness taper to your intended thickness (pure) reading.
Necks can also vary in form (with taper). An indicator will not press out that form against a pilot.