VERY nice button. I think it's coat and not cuff. Don't know rarity or value, but I've been hunting for many years and I have only found a few. If you can see gold gilt, drop it into a a little vinegar, let it soak for about 5 minutes and lightly brush it with a toothbrush to brighten the gold that's left, then rinse in water. If you can't see any gold, leave it like it is.