I wonder where they are coming from with that? Have they found some entries in the U.S. purchases book? Or is it a combination of Federal and CSA purchases of some specific type?

Or is it an error based on someone misreading the chamber size of a 12 pounder field howitzer (which is 3.67", 6 pounder--while the bore is that of a 12 pounder.) I'm leaning towards this latter.