I always group the buttons with eagles, feathers, crowns, etc together in my collection, all of them are blank on the front side so displaying them as b/m types is the way to go on them.
Here is a sentence or two from Tice's Dating Buttons books on this type button and their backmarks.
As the 1790s neared their end, some English makers used backmarks------containing decorative elements such as laurel wreaths. Beginning around 1800, backmarks often included icons such as eagles, feathers, and crowns. The use of eagles and the U.S. Navy device in backmarks was a ploy by English manufacturers appealing to American patriotism.
Basically when I seen plain faced buttons with eagles and the other devices mentioned I consider them 1800-1820era.
Don