I've not seen that particular style - looks like it may have been reused with a different diameter mortar - but hopper mortars are those that would have used some sort of basket enclosure around the perimeter of the mortar area to hold in the milled materials. This compound milling technique was more frequently used while the mortar hole was small. There seems to be some belief that with the deeper mortars the use of a containing basket was not necessary.

I'll see what I can dig up for references when I have a chance.

Greetings FireArc,
I agree that this is not a "hopper mortar," as there are no baskets (or evidence) associated with them. I simply used that term for lack of another descriptive term.

The use of a basket could have been only temporary - sometimes you can find asphaltum rings on the bedrock associated with their use, but they dont always have to be affixed to the surface.

As far as reuse is concerned, I would speculate that the deeper hole was made secondary to the wider one. In fact, I think the ledge created by the larger diameter hole would have proven useful by keeping some of the grist away from the mill while it works away in the lower hole.

What floral materials are available in the area? There are swab kits available that will allow you to take residue samples from artifacts such as these and have the contents analyzed. If you have that opportunity to do that, do it. The results should prove enlightening.

This looks similar to mortars that I saw at the Lava Beds National Monument when I was a kid. The area that they were located at is now closed to the public so I'm not sure if they are still there or not. I don't recall any of them exhibiting the differing size in circumference, but some of them were extremely deep, which we interpreted as reflecting years of use. They were in huge rocks, usually located at the base of juniper trees.

Thought you might be able to contact their archy and see if he has seen them.