Sturmtiger super-heavy 380mm assault mortar. Excellent tools for knocking down buildings.
Having taken a closer look at them while stripping off the old paint, it seems pretty likely to me that these are somebody's pirated models. The casting is very crude and blobby, especially around the running gear, but there is evidence of what was once some nice, fine detail, especially on the engine deck. I don't know who the original manufacturer might have been.