The unmilled totally. And the milled is only milled to a certain part. I actually have one milled for a woodvil that's milled less. I think what really matters is the length of your 510 to 510 adapter. Both will catch on the top of the shell, it just depends on the depth you require.
Basically, if it's a shorter adapter, the milled would be perfect. A little longer, than a woodvil milled would probably be best. If it puts the connector almost level, as if there was no well? You'd want an unmilled or "low profile" rm2