I understand that there is usually a great deal of texture to the surface of a mill roller, and you're probably right, but I don't think the suggestion it is a mill can be completely ruled out. I have one for barley which I bought, which is like you say, textured, but before that, I and a guy from work built one with smooth rollers we acquired from a scrap bin, and after just a minute amount of roughing the surface, it had all the grip it needed. It doesn't take as much surface texture as new mills currently have, just a few minutes with a wire wheel, something you wouldn't be able to tell through all that rust.