Hello
Only today did I realize that the current discussion on the misterious gun had originated from this message.

> The mystery in my mind is that they are stand behind a large naval cannon that was supposed to have been used by the Confederates during the battle.
> [...]
> Now if the caption of the picture is correct then this gun is that second known surviving artillery piece from the Battle of Arkansas Post.
> Appearently it was also recovered from the river and moved at some point back to Arkansas Post.

See the picture https://ibb.co/ZVW4C6t
I superimposed a semi-transparent scan from Olmstead, Stark & Tucker (page 33). As you can see, although the bottom part of the gun is not clearly visible, the two images match fairly closely. Look at the trunnion.
For check, I have added a six-foot scale to the right: although the exact line where the veteran's feet lay is not clear, generally the gun size is compatible with human size.

So, I believe that the gun depicted is a "32-pounder of 60 cwt, Gradual Increase", a model cast between 1817 and 1828. A fairly old piece by ACW time. It is exactly the same model as the two guns currently on display at St Charles (AR). However, the latter two are rifled, exactly like the couple which fought there on 17 June 1862 (disabling U.S.S. Mound City). I wonder if they are the same.

Deductions:

1) Either the caption of the picture is incorrect, or anything written about Fort Hindman needs a revision (I am for the former). No source I'm aware of shows a 32pdr there.

2) Initially I supposed that the veteran's reunion was actually a St Charles one, maybe celebrating the recovery of the guns that eventually are on display today. But I am told this happened later than 1920.

3) To be more precise, we should know more about the war record of Felix Allen from Casscoe (not my field of expertise. Anyone?).