22mm, Louisiana Coat Button with a "Horstmann & Allien/NY" backmark.  This is an excellent and published example, being the exact button pictured in Mike O-Donnell and Howard Crouch's book,  The Hammond Civil War Collection, top of P. 314, center  left of montage of seven buttons.  It is from the Richard M. Hammond colletion. It shows brighter even in person than the photo suggests.  I assure you that you will be very happy with this button or I will pay for postage both ways!