I may have found something relevant to answering my question about those smaller pieces, in Guddat's revised edition on Frederick the Great's artillery. The Prussian army had a small, light mortar, called a Coehoorn--perhaps after the Dutch-born general and siege warfare expert, Menno van Coehoorn? It had a caliber of 10-13cm, and weighed around 25 kilos, and was mounted on a small base that could be carried by two soldiers. And if the Prussians had them, I suspect that they were in use in other armies, too. The description paints a picture of a piece similar to those in the photo.
Prost!
Brad