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.  This button is inn good excavated condition and is hard to find and even more so in decent condition, as the pewter does not hold up well in Virginia soils. This is a bold, distinctive button with a great history.  This individual specimen was found on private property associated with the battle of Spotsylvania, Va