Layered Materials - Getting Alpha Channels

I have a height map in the base color of a texture. I have been unsuccessfully trying various ways to access it in a layer blend to do a height lerp based on the layer below it. Is it possible to get specifically the alpha channel (or any other arbitrary texture) from another layer?

I thought I could do a Component Mask with just the Alpha but it does not like that these are V3.

Any other ideas?