Many civilian sport-coat/blazer buttons have been manufactured which imitate the design of a US Navy button. A simple way to tell the difference (without even looking at the back) is that the civilian-clothing imitations have "weaker" detail in the emblem. Meaning, details like the "lined background" and feathers are not crisp/sharply-defined, whereas actual US Navy buttons are quite sharply-detailed. Your button appears to be a civilian-clothing imitation.