Hello Keith,
You are correct about the Alger 6 pounder Rifled smoothbore at Fredericksburg. It was a Alger M1841 6 pounder field gun built in 1853 but altered later into a James Rifle, Type I 3.8" bore. The foundry number is 468 and the letters are the initials of the inspector, Louis A. DeBarth Walbach. This cannon is supposed to have an inscription "L B Davies" who was Lorenzo B Davies, an official from Ohio. This was not built by Revere as you suggest in your post but by Alger in Boston MA. This gun is probably in good condition because it was not used much during the civil war. The guns of this type were more experimental and developmental and when retired to storage as the Ordnance Dept advanced to a newer model of James Rifles. When cannon were taken out of storage 25 years later for the new military parks, there they were ready and in good condition. The wood gun carriages used in the civil war were replaced by iron gun carriages built at time of formation of the parks.

The other field gun you mention, at Spotsylvania, is possibly a False Napoleon. If so, this work was done in Gettysburg PA by Capt. Gilbert to fill need for ore napoleon guns for the new parks. It was partially reamed to a larger diameter, in the first four inches as you say, also about the time of the formation of the military parks. Your description and the fact the barrel is a abrupt transition from the originial bore diameter to the larger bore modification was common when a gun was altered for the parks. This was how they altered a field gun to appear as another. I believe, from your information, to be a bronze 6 pounder field gun built in 1847 by Ames Foundry in Springfield MA. When field guns were altered, it was possible for stampings to be erased in the process. The gun registry number does not appear in your description as well as the manufacturers name. Springfield identifies this gun as Ames built. Wayne Stark's list, now operated by Jim Bender, lists a Ames M1841 at Hazel Grove but it was built in 1842 with registry number 43.

This is the best I can provide from here with the info you sent.
Thanks Keith
Ron