I agree, it sure looks like one. Out of curiousity though are you certain that it's a snap button? The reason I ask is that there was a "Bob White Brand" of foods in the same era as this (looks 1920's-30's to me). And this also has somewhat of the same appearances of a cap to a tube of something, like tomato paste if it were from the food company. But if the back has no threads like a cap, then it's not that, and I'd totally agree with work clothes button. I couldn't find a matching button or clothing reference to that brand yet though.