The trigger guard has a fitting for a loop for a sling and the barrel band has a fitting and loop. While a sling would be useful on a long gun, it would be useless on a pistol. However, it does indicate that the trigger guard came off of a military firearm. While some Revolutionary War long arms had a loop mounted on the trigger guard, the U.S. model 1795 and first model 1816 muskets had a separate mount just in front of the trigger guard. As more model 1816's were produced the mount was changed to the trigger guard itself. The hammer appears to be of a style used on older weapons such as Revolutionary War period muskets and early long rifles. It has a "swan neck" appearance rather than the wider version with heart shaped cutout seen on U.S. military muskets. The lock plate is flat with two vertical lines behind the hammer. This style is often seen on long rifles; military weapons frequently have a lock plate with a curved surface, think banana rather than flat. The barrel band is backwards from the way it typically appears on military long arms. The square in its side is for a spring to hold it in place, but there is no spring. The barrel looks like it might be held to the stock by pins (not the hole in the stock under the barrel between the band and the muzzle of the weapon.) The barrel seems thicker than for typical muskets. The lock bolt plate is triangular which is more like a long rifle or fowling gun. Military weapons, both long guns and hand guns usually have two circular ends connected by an L-shaped or diagonal piece for their lock bolt plate. Looking at the barrel and the tang which attaches to the stock, you can see that the tang was once separated from the barrel and screwed back on. There are two notches at the base of the barrel that don't quite match up with the two notches on the tang. On the butt of the pistol are incised lines that indicate where a butt cap should be. Instead of a butt cap there appears to be darkened color which is partially rubbed off in spots. I can't tell if this coloring is from oxidation of a metal butt cap that has disappeared or a faux butt cap to appear as if there is a metal one.
My guess is that this weapon is a hodge-podge of parts, or franken-pistol. There are various reasons and times this might be done. If it was done to make a functioning firearm, this suggests that there was someone with limited resources. Examples could be people on the frontier trying to make working firearms out of parts from damaged firearms or Native Americans trying to come up with a firearm. I don't see any Native American style decoration added and it seems to be in reasonable condition which would tend to argue against Native Americans but that's not a deal breaker. I would think a gunsmith would not put a barrel band on backwards and would probably have the correct spring to keep it in place, but I suppose that is not a deal breaker either. It could be a Civil War period or later composite weapon put together in Mexico, there are a lot of composite swords that were put together made out of pieces parts of different swords in Mexico from that time. Two other scenarios that I could come up with are that it was put together as a movie prop between about 1890 and 1960, or it was cobbled together by an antique dealer trying to make a buck. Any of these possible histories and others I can't even imagine could end up placing it in a field in Kansas, but the origin story is uncertain so there is no firm time and place where it must show up.
This being said, I am not a firearms expert by any means. There are plenty of others with far more knowledge and experience who can probably tell you much more with more certainty than I can.
Bringing the