23mm, Chassseur's button. These buttons were awarded on the uniforms given out to the several regiments that made the best showing at a grand review of the Army of the Potomac held during the summer of 1862.  If memory serves me correctly the Grand Review was held at Harrison's Landing (better check me on that detail, as it could have been next door).  Anyway, American residents of Paris, France raised the money to supply uniforms and equipage for several regiments and sent these French fashion uniforms over early in the war.  George B. McClellan made the award of these buttons and uniforms.