that is a nice flat button you got there. you must be hunting an old spot. i have only found one flat button but the marking on the back is worn off to a point were i cant distinguish what the maker of it was.
Jake is right, try to get a read on that maker. it looks pretty good, at least good enough to read and look up the company/maker and it should tell you when that company was in business making those buttons.
HH and nice find, lance