As many folks in this forum already know, collectors need to be extra-careful about 2-piece and 3-piece buttons which have a "Quality" backmark. There are many variations of such backmarks -- and a significant portion of the variations are post-1865 era.
That being said... on your button, the specific version of "Extra Quality" backark (noting the letter-font, the letter-size, the additional non-letter markings such as stars, etc) indicates your button is civil war era.