Jim,
The first gun is a M1857 light field gun as described by George Martin. The description of the four rifled guns, bronze 6 pounder James 3.8"rifles contains a error. A 3.8" James rifle was never a 6 pounder rifled field gun, it was a 14 pounder field gun. Smoothbore field guns were described using a number for the pounder size of the gun. The problem was that rifled field guns actually were described by the size of the bore and in this case, the 3.8" bore of the James rifle you ask about is the correct description for a rifled piece. In a smoothbore field piece, these guns fired a projectile that actually weighed the amount mentioned in the description, such as the M1841 field gun fired a 6 pound ball thus it was called a 6 pounder field piece. Rifled pieces were known by the bore of the tube. Example the M1861 Parrot rifled field piece fired a 2.9" projectile out of a barrel of 2.9" size.
The descriptions mentioned by the general, are too general and more information is needed to determine what type they were. Finally, Yes, the 12 pound light pieces may actually be the 12 pounder mountain howitzer you mention

A note on nomenclature of Cannon, the word pounder is used to description of the gun and not the word pound. There is a 6 pounder field piece but never a 6 pound field piece.

Hope this helps
Ron