Don,
I was wondering the same thing as Artilleryman. Do you have another pic of the muzzle which might reveal more than the inspector's initials we can see here, CCC? He inspected these Ordinance Rifles which were made at Phoenix Iron Works in Phoenixville, PA. during the war years. According to the National List of Surviving Civil War Artillery formerly compiled by the late Wayne Stark and others, this gun should be 816 Lbs. with a registry No. of 528 and a Foundry No. of 562, made in 1862. Having right-hand rifling of a 7 groove design, these field guns were well thought of by the men who were assigned to serve them.
Thank you for including the photo of the plaque. Without this, most of us would not be aware of this gun's extremely unique story.
Mike and Tracy
P.S. I just found the inspector's name who was the Union Army's ordinance inspector at the Phoenix Iron Works from 1862 to 1867; it is Clemens C. Chafee.