Bedrock Bryan wrote:
> My experience says one piece buttons are usually at least pre 1860s or so.
Well... that's true, but "usually" is a long way from always. A great many 1-piece metal buttons have been manufactured in the Modern Era, for Fashion clothing, and they can easily be found by examining the buttons on various coats and jackets at any major Clothing store.
Most often, the modern-era metal 1-piece buttons are made of various cheap "White-Metal" alloys (such as pot-metal, or aluminum, or zinc). Also, they tend to have a "tombstone" shank, with a hole drilled through it. That type of shank seems to be visible on your 1-piece white-metal eagle button. Also, note that it appears to have a perfectly flat back, which is another sign of a modern-era 1-piece button.
Please clean all the dirt off your button's back, and post several photos showing it, shot from different angles. Thanks in advance for the effort.