Best I can tell you from the pics is that it's a boot pistol probably Belgian made There were a lot of makers that made pistols just like it. Date wise 1830's to around 1870ish maybe.
About 6” down in farm field not off beaten track. But read like iron. So probably just passed over many times. I saw the end of the barrel first and thought I had a piece of pipe.
Boy was I shocked when I pulled it out.
Scrub all the loose rust off, soak it in evaporust for a while and clear the barrel completely (make sure it's not loaded) then boil it in paraffin wax for a few minutes (bake it in the oven to make sure all the moisture is out first) when it cools buff it with a soft cloth and it will make a great display and last a long time without corroding any further