Complex sentence: The origins and exact age of the Fleming Cannon are not known with certainty, but it is believed to have been cast during the Franco-Prussian War era, but completed in 1878 after the war was over.
Split into simpler sentences: The origins and exact age of the Fleming Cannon are not known with certainty. It is believed to have been cast during the Franco-Prussian War era, but completed in 1878 after the war was over.