It's a Cufflink! It's a button! It's both! And it holds your sleeve shut to boot!
NOTE: These DO NOT come with thread in the holes (as you can see in the 3D file viewer). I did that after to add a little flair, and it's super easy to do. I'd be happy to help you with the process. For more on my designs or to contact me, visit www.prero.com.
ALSO NOTE: The "GP" on the back has changed in the most recent file. Aside form that it's the same.