This is a CPO shirt button from the GEMSCO company of the US Navy, believed to be from the 1930s. It's an old type of button without sharp corners. It appears to be unused. There are 114 buttons in total, with slight variations in individual size and design. The approximate diameter is 16.5mm to 17.5mm. These were previously found in a sailor's suitcase that was acquired in the United States. I believe this is a very rare item. Thank you.