In all probability they were intended to train the crews of large seacoast mortars in the execution of indirect fire, in order to save the expense of firing large shells.
There are some similarities between the two Models of 1890, breech loaders that had to be lowered to horizontal to load. But differences, too. 20 lb shell versus 800 or 1000 lb shell requiring mechanical loading, no hydraulics, etc.
Would kind of like to make a subscale version of the 12" mortar also. Need good plans for both. Should give AOP a call to see if he has anything.