It is definitely a 20th Century civilian coat "blazer" button. Even though it has an official-looking emblem (royal crown above a shield with rampant lion), the low-quality detailing and rusted cheap iron/steel back mean it is a modern civilian-usage button. These regal-looking metal buttons are used to make the coat appear to be "pricier quality" than plain plastic buttons would indicate. Go to any store that sells men's coats (even WalMart) and you'll see many similar metal buttons.