The way the back looks, it looks like someone just pulled the shank out. It left a hole. The button is 18.54 millimeters wide. Vest size? I can post a pic of the back but it is so corroded that the letters are gone. Just a hole back there. There are three ( I think that it's three) cannonballs stacked by the front point of the anchor. I have found a couple of buttons that look like it a couple of the online relic stores. It was found at an old homesite that was supposed to have been used after the Battle of Shiloh as a hospital. In fact, every house in Savannah TN (where I found it) was commandeered by US Grant to be used as a hospital as the wounded soldiers returned from the battle. But that doesn't make me 100% sure that it was worn by a Union Naval soldier in the Civil War. Just about 90%. We do have the Tennessee river running through town that was used by Federal gunboats at Shiloh (Pittsburg landing). It is within the realm of possibility that one of the navy soldiers left behind a button. Especially if he was wounded. Just speculation.