Mano (grinding) stone?

This stone is about 5" long x 3" wide x 2" high and has been ground perfectly flat on one side. Not sure if this is a stone tool or a remnant of some sort of dressed stone.

It was a field find. I have found a variety of chert and flint tools. arrowheads, scrapers, awls. etc., in Central Ilinois, but nothing like this. The stone is rough which makes it very easy to hold and it seems like a couple of areas have been relieved to make it fit the hand. It weighs 2-3 lbs.

Except for the flat surface it is unremarkable. It took some deliberate effort to create the flat surface, probably for use as a grinding stone (pestle) for processing food. The surface, in general, appears to be porous but it is quite dense and deceptively heavy for its size. One edge is deeply stained with some dark substance. That edge is very flat except for one minor recession, but the stain makes it appear to be uneven.

There is some sort of short grain, perhaps three feet tall, with a heavy seed bundle at the top, still growing where I found this stone. I may have taken photos of some of it. Will have to dig through my old photos to see.

Here is an image which may show how it was used.

https://en.m.wikipedia.org/wiki/Metate